Fix CI for darwin

nix-darwin is detecting `/etc/nix/nix.conf` from the
cachix/install-nix-action and refusing to overwrite it, failing the
nix-darwin activation and therefore the rest of CI.

This commit `mv`s the existing `nix.conf` to avoid the above, and then
adds support for new-style nix commands and flakes to the nix-darwin
configuration to allow their subsequent use in CI.

It also removes the nix config from the `cachix/install-nix-action`
step, which was duplicated effort since we are blowing it away with
nix-darwin anyway.

Relevant:

- https://github.com/LnL7/nix-darwin/issues/149

.github/workflows/ci.yaml

@@ -15,21 +15,29 @@ jobs:
     - run: nix fmt . -- --check
     - run: nix flake check
   tests-darwin:
-    runs-on: macos-11
+    runs-on: macos-12
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v22
+    - uses: cachix/install-nix-action@v24
       with:
-        extra_nix_config: "system-features = nixos-test benchmark big-parallel kvm"
+        extra_nix_config: |
+          system-features = nixos-test recursive-nix benchmark big-parallel kvm
+          extra-experimental-features = recursive-nix nix-command flakes
     - run: nix build
     - run: nix build .#doc
     - run: nix fmt . -- --check
     - run: nix flake check
     - name: "Install nix-darwin module"
       run: |
-        system=$(nix build --no-link --print-out-paths .#checks.x86_64-darwin.integration)
-        ${system}/activate-user
-        sudo ${system}/activate
+        # https://github.com/ryantm/agenix/pull/230#issuecomment-1867025385
+
+        sudo mv /etc/nix/nix.conf{,.bak}
+        nix \
+          --extra-experimental-features 'nix-command flakes' \
+          build .#checks.x86_64-darwin.integration
+
+        ./result/activate-user
+        sudo ./result/activate
     - name: "Test nix-darwin module"
       run: |
         sudo /run/current-system/sw/bin/agenix-integration

flake.nix

@@ -49,7 +49,9 @@
             inherit system;
             modules = [
               ./test/integration_darwin.nix
-              darwin.darwinModules.simple
+
+              # Allow new-style nix commands in CI
+              {nix.extraOptions = "experimental-features = nix-command flakes";}
 
               home-manager.darwinModules.home-manager
               {