installer: simplify the per-build installation

The goal is to allow the installation and testing of arbitrary Nix
versions. Extend the base installer to accept a `--tarball-url <url>` to
change where the Nix tarball is getting downloaded from.

Once this is merged it should allow to:
1. Pick an evaluation at https://hydra.nixos.org/jobset/nix/master that
   looks healthy
2. Select the installedScript build and find the store path.

Now equipped with all of this, use an instance of nar-serve to fetch the
install script and release tarballs:

    curl -sfL https://nar-serve.numtide.com/nix/store/rkv4yh7pym941bhj0849zqdkg2546bdv-installer-script/install \
      | sh --tarball-url https://nar-serve.numtide.com/nix/store

Or with cachix, strip the /nix/store and derivation name and then:

    curl -sfL https://mycache.cachix.org/serve/rkv4yh7pym941bhj0849zqdkg2546bdv/install \
      | sh --tarball-url https://mycache.cachix.org/serve

Fixes NixOS#4047

flake.nix

@@ -324,9 +324,26 @@
             ''
               mkdir -p $out/nix-support
 
+              # Converts /nix/store/50p3qk8kka9dl6wyq40vydq945k0j3kv-nix-2.4pre20201102_550e11f/bin/nix
+              # To 50p3qk8kka9dl6wyq40vydq945k0j3kv/bin/nix
+              tarballPath() {
+                # Remove the store prefix
+                local path=''${1#${builtins.storeDir}/}
+                # Get the path relative to the derivation root
+                local rest=''${path#*/}
+                # Get the derivation hash
+                local drvHash=''${path%%-*}
+                echo "$drvHash/$rest"
+              }
+
               substitute ${./scripts/install.in} $out/install \
                 ${pkgs.lib.concatMapStrings
-                  (system: "--replace '@binaryTarball_${system}@' $(nix --experimental-features nix-command hash-file --base16 --type sha256 ${self.hydraJobs.binaryTarball.${system}}/*.tar.xz) ")
+                  (system:
+                    '' \
+                    --replace '@tarballHash_${system}@' $(nix --experimental-features nix-command hash-file --base16 --type sha256 ${self.hydraJobs.binaryTarball.${system}}/*.tar.xz) \
+                    --replace '@tarballPath_${system}@' $(tarballPath ${self.hydraJobs.binaryTarball.${system}}/*.tar.xz) \
+                    ''
+                  )
                   [ "x86_64-linux" "i686-linux" "x86_64-darwin" "aarch64-linux" ]
                 } \
                 --replace '@nixVersion@' ${version}

scripts/install.in

@@ -25,18 +25,44 @@ require_util() {
 }
 
 case "$(uname -s).$(uname -m)" in
-    Linux.x86_64) system=x86_64-linux; hash=@binaryTarball_x86_64-linux@;;
-    Linux.i?86) system=i686-linux; hash=@binaryTarball_i686-linux@;;
-    Linux.aarch64) system=aarch64-linux; hash=@binaryTarball_aarch64-linux@;;
-    Darwin.x86_64) system=x86_64-darwin; hash=@binaryTarball_x86_64-darwin@;;
-    # eventually maybe: system=arm64-darwin; hash=@binaryTarball_arm64-darwin@;;
-    Darwin.arm64) system=x86_64-darwin; hash=@binaryTarball_x86_64-darwin@;;
+    Linux.x86_64)
+        hash=@tarballHash_x86_64-linux@
+        store_path=@tarballPath_x86_64-linux@
+        system=x86_64-linux
+        ;;
+    Linux.i?86)
+        hash=@tarballHash_i686-linux@
+        store_path=@tarballPath_i686-linux@
+        system=i686-linux
+        ;;
+    Linux.aarch64)
+        hash=@tarballHash_aarch64-linux@
+        store_path=@tarballPath_aarch64-linux@
+        system=aarch64-linux
+        ;;
+    Darwin.x86_64)
+        hash=@tarballHash_x86_64-darwin@
+        store_path=@tarballPath_x86_64-darwin@
+        system=x86_64-darwin
+        ;;
+    Darwin.arm64)
+        # eventually maybe: system=arm64-darwin
+        system=x86_64-darwin
+        store_path=@tarballPath_x86_64-darwin@
+        hash=@binaryTarball_x86_64-darwin@
+        ;;
     *) oops "sorry, there is no binary distribution of Nix for your platform";;
 esac
 
-url="https://releases.nixos.org/nix/nix-@nixVersion@/nix-@nixVersion@-$system.tar.xz"
+# Use this command-line option to fetch the tarballs using nar-serve or Cachix
+if "${1:---tarball-url}"; then
+    url=${2}/${store_path}
+    shift 2
+else
+    url=https://releases.nixos.org/nix/nix-@nixVersion@/nix-@nixVersion@-$system.tar.xz
+fi
 
-tarball="$tmpDir/$(basename "$tmpDir/nix-@nixVersion@-$system.tar.xz")"
+tarball=$tmpDir/nix-@nixVersion@-$system.tar.xz
 
 require_util curl "download the binary tarball"
 require_util tar "unpack the binary tarball"