ci: Move magic-nix-cache-action into install-nix-action composite

This reduces duplication and pins the underlying version of magic-nix-cache,
as we already do with other actions.

.github/actions/install-nix-action/action.yaml

@@ -23,6 +23,10 @@ inputs:
   github_token:
     description: "Github token"
     required: true
+  use_cache:
+    description: "Whether to setup magic-nix-cache"
+    default: true
+    required: false
 runs:
   using: "composite"
   steps:
@@ -118,3 +122,10 @@ runs:
         source-url: ${{ inputs.experimental-installer-version != 'latest' && 'https://artifacts.nixos.org/experimental-installer/tag/${{ inputs.experimental-installer-version }}/${{ env.EXPERIMENTAL_INSTALLER_ARTIFACT }}' || '' }}
         nix-package-url: ${{ inputs.dogfood == 'true' && steps.download-nix-installer.outputs.tarball-path || (inputs.tarball_url || '') }}
         extra-conf: ${{ inputs.extra_nix_config }}
+    - uses: DeterminateSystems/magic-nix-cache-action@565684385bcd71bad329742eefe8d12f2e765b39 # v13
+      if: ${{ inputs.use_cache == 'true' }}
+      with:
+        diagnostic-endpoint: ''
+        use-flakehub: false
+        use-gha-cache: true
+        source-revision: c2f46a0afa5f95fd4c184a533afd280c68cf63ff # v0.1.6

.github/workflows/ci.yml

@@ -29,6 +29,7 @@ jobs:
         extra_nix_config:
           experimental-features = nix-command flakes
         github_token: ${{ secrets.GITHUB_TOKEN }}
+        use_cache: false
     - run: nix flake show --all-systems --json
 
   pre-commit-checks:
@@ -41,7 +42,6 @@ jobs:
           dogfood: ${{ github.event_name == 'workflow_dispatch' && inputs.dogfood || github.event_name != 'workflow_dispatch' }}
           extra_nix_config: experimental-features = nix-command flakes
           github_token: ${{ secrets.GITHUB_TOKEN }}
-      - uses: DeterminateSystems/magic-nix-cache-action@main
       - run: ./ci/gha/tests/pre-commit-checks
 
   basic-checks:
@@ -92,7 +92,6 @@ jobs:
         dogfood: ${{ github.event_name == 'workflow_dispatch' && inputs.dogfood || github.event_name != 'workflow_dispatch' }}
         # The sandbox would otherwise be disabled by default on Darwin
         extra_nix_config: "sandbox = true"
-    - uses: DeterminateSystems/magic-nix-cache-action@main
     # Since ubuntu 22.30, unprivileged usernamespaces are no longer allowed to map to the root user:
     # https://ubuntu.com/blog/ubuntu-23-10-restricted-unprivileged-user-namespaces
     - run: sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0
@@ -230,7 +229,6 @@ jobs:
     - uses: cachix/install-nix-action@v31
       with:
         install_url: https://releases.nixos.org/nix/nix-2.20.3/install
-    - uses: DeterminateSystems/magic-nix-cache-action@main
     - run: echo NIX_VERSION="$(nix --experimental-features 'nix-command flakes' eval .\#nix.version | tr -d \")" >> $GITHUB_ENV
     - run: nix --experimental-features 'nix-command flakes' build .#dockerImage -L
     - run: docker load -i ./result/image.tar.gz
@@ -289,7 +287,6 @@ jobs:
           extra_nix_config:
             experimental-features = nix-command flakes
           github_token: ${{ secrets.GITHUB_TOKEN }}
-      - uses: DeterminateSystems/magic-nix-cache-action@main
       - run: nix build -L --out-link ./new-nix && PATH=$(pwd)/new-nix/bin:$PATH MAX_FLAKES=25 flake-regressions/eval-all.sh
 
   profile_build:
@@ -310,7 +307,6 @@ jobs:
         extra_nix_config: |
           experimental-features = flakes nix-command ca-derivations impure-derivations
           max-jobs = 1
-    - uses: DeterminateSystems/magic-nix-cache-action@main
     - run: |
         nix build -L --file ./ci/gha/profile-build buildTimeReport --out-link build-time-report.md
         cat build-time-report.md >> $GITHUB_STEP_SUMMARY