Add home-manager instantiate subcommand

It can be useful to simply instantiate a home-manager configuration
without actually building it, for example for the purpose of
pre-building it with some custom command.

home-manager/home-manager

@@ -74,6 +74,7 @@ function setHomeManagerNixPath() {
     done
 }
 
+
 function doBuildAttr() {
     setConfigFile
     setHomeManagerNixPath
@@ -89,7 +90,14 @@ function doBuildAttr() {
     fi
 
     # shellcheck disable=2086
-    if [[ -v USE_NIX2_COMMAND ]]; then
+    if [[ -v INSTANTIATE ]]; then
+        nix-instantiate \
+            "<home-manager/home-manager/home-manager.nix>" \
+            "${extraArgs[@]}" \
+            "${PASSTHROUGH_OPTS[@]}" \
+            --argstr confPath "$HOME_MANAGER_CONFIG" \
+            --argstr confAttr "$HOME_MANAGER_CONFIG_ATTRIBUTE"
+    elif [[ -v USE_NIX2_COMMAND ]]; then
         nix build \
             -f "<home-manager/home-manager/home-manager.nix>" \
             "${extraArgs[@]}" \
@@ -182,6 +190,21 @@ function doBuild() {
     return $exitCode
 }
 
+function doInstantiate() {
+    local exitCode
+
+    INSTANTIATE=1
+    if [[ -v USE_NIX2_COMMAND ]]; then
+        doBuildAttr activationPackage \
+            && exitCode=0 || exitCode=1
+    else
+        doBuildAttr --attr activationPackage \
+            && exitCode=0 || exitCode=1
+    fi
+
+    return $exitCode
+}
+
 function doSwitch() {
     setWorkDir
 
@@ -471,7 +494,7 @@ while [[ $# -gt 0 ]]; do
     opt="$1"
     shift
     case $opt in
-        build|edit|expire-generations|generations|help|news|packages|remove-generations|switch|uninstall)
+        build|instantiate|edit|expire-generations|generations|help|news|packages|remove-generations|switch|uninstall)
             COMMAND="$opt"
             ;;
         -2)
@@ -542,6 +565,9 @@ case $COMMAND in
     build)
         doBuild
         ;;
+    instantiate)
+        doInstantiate
+        ;;
     switch)
         doSwitch
         ;;