ci: use the PR head as <darwin> for install test

This was installing against the base branch, which causes annoying
issues when things like the state version are bumped. It’s possible
this was intentional to test the upgrade path, but it’s inconsistent
with the flake installation test and keeps breaking stuff on my
PRs. If we want to test the upgrade path then we could re‐add it
again later in a more sophisticated manner (e.g. using the example
config from the previous version).

.github/workflows/test.yml

@@ -7,7 +7,6 @@ on:
 
 env:
   NIXPKGS_BRANCH: nixpkgs-unstable
-  NIX_DARWIN_BRANCH: master
   NIX_VERSION: 2.24.11
 
 jobs:
@@ -40,7 +39,6 @@ jobs:
         nix_path: nixpkgs=channel:${{ env.NIXPKGS_BRANCH }}
     - name: Install channels
       run: |
-        nix-channel --add https://github.com/LnL7/nix-darwin/archive/${{ env.NIX_DARWIN_BRANCH }}.tar.gz darwin
         nix-channel --add https://nixos.org/channels/${{ env.NIXPKGS_BRANCH }} nixpkgs
         nix-channel --update
     - name: Install nix-darwin
@@ -52,11 +50,16 @@ jobs:
 
         nixConfHash=$(shasum -a 256 /etc/nix/nix.conf | cut -d ' ' -f 1)
         /usr/bin/sed -i.bak \
-          "s/# programs.fish.enable = true;/nix.settings.access-tokens = [ \"github.com=\${{ secrets.GITHUB_TOKEN }}\" ]; environment.etc.\"nix\/nix.conf\".knownSha256Hashes = [ \"$nixConfHash\" ];/" \
+          "s/# programs.fish.enable = true;/ \
+            nix.settings.access-tokens = [ \"github.com=\${{ secrets.GITHUB_TOKEN }}\" ]; \
+            environment.etc.\"nix\/nix.conf\".knownSha256Hashes = [ \"$nixConfHash\" ]; \
+            nix.nixPath = lib.mkBefore [ { darwin = \"${PWD////\/}\"; } ]; \
+          /" \
           ~/.config/nix-darwin/configuration.nix
 
         nix run .#darwin-rebuild \
           -- switch \
+          -I darwin=. \
           -I darwin-config=$HOME/.config/nix-darwin/configuration.nix
     - name: Switch to new configuration
       run: |
@@ -66,7 +69,7 @@ jobs:
           "s/pkgs.vim/pkgs.hello/" \
           ~/.config/nix-darwin/configuration.nix
 
-        darwin-rebuild switch -I darwin=.
+        darwin-rebuild switch
 
         hello
     - name: Test uninstallation of nix-darwin