feat: cache eval nix store to speed up evaluation job

Author: jfroche

.github/actions/nix-install-ephemeral/action.yml

@@ -5,9 +5,21 @@ inputs:
     description: 'Whether to push build outputs to the Nix binary cache'
     required: false
     default: 'true'
+  restore-nix-store:
+    description: 'Whether to restore the Nix store from the binary cache before building'
+    required: false
+    default: 'false'
+  restore-cache-name:
+    description: 'The name to use for the Nix store cache (if restoring)'
+    required: false
 runs:
   using: 'composite'
   steps:
+    - name: Restore and save Nix store
+      uses: nix-community/cache-nix-action@v6
+      with:
+        primary-key: nix-${{ inputs.restore-cache-name }}-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+        restore-prefixes-first-match: nix-${{ inputs.restore-cache-name }}-${{ runner.os }}-${{ runner.arch }}-
     - name: aws-creds
       uses: aws-actions/configure-aws-credentials@v4
       if: ${{ inputs.push-to-cache == 'true' }}

.github/workflows/nix-eval.yml

@@ -25,8 +25,11 @@ jobs:
         env:
           DEV_AWS_ROLE: ${{ secrets.DEV_AWS_ROLE }}
           NIX_SIGN_SECRET_KEY: ${{ secrets.NIX_SIGN_SECRET_KEY }}
+        with:
+          restore-nix-store: true
+          restore-cache-name: nix-eval
       - id: set-matrix
         name: Generate Nix Matrix
         run: |
           set -Eeu
-          echo matrix="$(nix run .\#github-matrix checks legacyPackages)" >> "$GITHUB_OUTPUT"
+          echo matrix="$(nix run --accept-flake-config .\#github-matrix checks legacyPackages)" >> "$GITHUB_OUTPUT"