Add nix-community/cache-nix-action for Nix store caching

Replaces the dropped magic-nix-cache-action. Uses GitHub Actions cache
to persist /nix between runs, keyed by hashes of all .nix files plus
flake.lock so a flake change invalidates the cache. Falls back to the
most recent same-OS cache via restore-prefixes-first-match.

Per-OS GC ceilings (5G linux, 3G macOS) keep cached store size under
GitHub's 10G per-repo limit while leaving room for other caches.

Author: ananthb

.github/workflows/ci.yml

@@ -13,6 +13,12 @@ jobs:
 
       - uses: DeterminateSystems/nix-installer-action@main
 
+      - uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+          gc-max-store-size-linux: 5G
+
       - name: Build
         run: nix build
 

.github/workflows/release.yml

@@ -14,6 +14,12 @@ jobs:
 
       - uses: DeterminateSystems/nix-installer-action@main
 
+      - uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+          gc-max-store-size-linux: 5G
+
       - name: Build release tarball
         run: |
           nix build .#packages.x86_64-linux.release -o result-release
@@ -35,6 +41,12 @@ jobs:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
 
+      - uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+          gc-max-store-size-macos: 3G
+
       - name: Build package
         run: nix build .#packages.aarch64-darwin.default -o result