nixos-anywhere: allow users to specify custom path NixOS configurations

This is useful for users who use a similar `extendModules` pattern like
`virtualisation.vmVariant`, where an extended NixOS configuration lives
inside an existing configuration:

    nixos-anywhere --flake .#nixosConfigurations.alpha.config.virtualisation.vmVariant

src/default.nix

@@ -11,6 +11,7 @@
 , findutils
 , gnused
 , sshpass
+, jq
 , terraform-docs
 , lib
 , makeWrapper
@@ -29,6 +30,7 @@ let
     gnused # needed by ssh-copy-id
     sshpass # used to provide password for ssh-copy-id
     gnutar # used to upload extra-files
+    jq # used for checking if flakeAttr exists
   ];
 in
 stdenv.mkDerivation {

src/nixos-anywhere.sh

@@ -313,6 +313,20 @@ parseArgs() {
       echo 'For example, to use the output nixosConfigurations.foo from the flake.nix, append "#foo" to the flake-uri.' >&2
       exit 1
     fi
+
+    # We check if nixosConfigurations."<flakeAttr>".config.system.build.toplevel exists
+    # to allow users to use the shorthand $flake#foo. We use `nix flake metadata $flake`
+    # to add $flake to the store, otherwise relative paths will fail when used with
+    # `builtins.getFlake`. We use `nix eval --expr ... ? toplevel` to avoid instantiating
+    # the derivation as this may require building the entire system if the user uses IFD
+    # like through `system.replaceDependencies`.
+    if [[ $(nix eval \
+              --expr \
+              "(builtins.getFlake \"$(nix flake metadata "$flake" --json | jq -r '.path')\").nixosConfigurations.\"$flakeAttr\".config.system.build ? toplevel" \
+              --impure 2> /dev/null) == "true" ]];
+    then
+      flakeAttr="nixosConfigurations.\"$flakeAttr\".config"
+    fi
   fi
 
 }
@@ -363,7 +377,7 @@ runVmTest() {
     --no-link \
     -L \
     "${nixOptions[@]}" \
-    "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.installTest"
+    "${flake}#${flakeAttr}.system.build.installTest"
 }
 
 uploadSshKey() {
@@ -539,11 +553,11 @@ runDisko() {
     step Building disko script
     # We need to do a nix copy first because nix build doesn't have --no-check-sigs
     # Use ssh:// here to avoid https://github.com/NixOS/nix/issues/7359
-    nixCopy --to "ssh://$sshConnection" "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.${diskoMode}Script" \
+    nixCopy --to "ssh://$sshConnection" "${flake}#${flakeAttr}.system.build.${diskoMode}Script" \
       --derivation --no-check-sigs
     # If we don't use ssh-ng here, we get `error: operation 'getFSAccessor' is not supported by store`
     diskoScript=$(
-      nixBuild "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.${diskoMode}Script" \
+      nixBuild "${flake}#${flakeAttr}.system.build.${diskoMode}Script" \
         --eval-store auto --store "ssh-ng://$sshConnection?ssh-key=$sshKeyDir/nixos-anywhere"
     )
   fi
@@ -561,11 +575,11 @@ nixosInstall() {
     step Building the system closure
     # We need to do a nix copy first because nix build doesn't have --no-check-sigs
     # Use ssh:// here to avoid https://github.com/NixOS/nix/issues/7359
-    nixCopy --to "ssh://$sshConnection?remote-store=local?root=/mnt" "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.toplevel" \
+    nixCopy --to "ssh://$sshConnection?remote-store=local?root=/mnt" "${flake}#${flakeAttr}.system.build.toplevel" \
       --derivation --no-check-sigs
     # If we don't use ssh-ng here, we get `error: operation 'getFSAccessor' is not supported by store`
     nixosSystem=$(
-      nixBuild "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.toplevel" \
+      nixBuild "${flake}#${flakeAttr}.system.build.toplevel" \
         --eval-store auto --store "ssh-ng://$sshConnection?ssh-key=$sshKeyDir/nixos-anywhere&remote-store=local?root=/mnt"
     )
   fi
@@ -626,9 +640,9 @@ main() {
   if [[ -n ${flake} ]]; then
     if [[ ${buildOnRemote} == "n" ]] && [[ ${hardwareConfigBackend} == "none" ]]; then
       if [[ ${phases[disko]} == 1 ]]; then
-        diskoScript=$(nixBuild "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.${diskoMode}Script")
+        diskoScript=$(nixBuild "${flake}#${flakeAttr}.system.build.${diskoMode}Script")
       fi
-      nixosSystem=$(nixBuild "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.toplevel")
+      nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
     fi
   elif [[ -n ${diskoScript} ]] && [[ -n ${nixosSystem} ]]; then
     if [[ ! -e ${diskoScript} ]] || [[ ! -e ${nixosSystem} ]]; then
@@ -684,9 +698,9 @@ main() {
 
   if [[ ${buildOnRemote} == "n" ]] && [[ -n ${flake} ]] && [[ ${hardwareConfigBackend} != "none" ]]; then
     if [[ ${phases[disko]} == 1 ]]; then
-      diskoScript=$(nixBuild "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.${diskoMode}Script")
+      diskoScript=$(nixBuild "${flake}#${flakeAttr}.system.build.${diskoMode}Script")
     fi
-    nixosSystem=$(nixBuild "${flake}#nixosConfigurations.\"${flakeAttr}\".config.system.build.toplevel")
+    nixosSystem=$(nixBuild "${flake}#${flakeAttr}.system.build.toplevel")
   fi
 
   # Installation will fail if non-root user is used for installer.