CI/Nix: add cache-nix-action

Use nixbuild/nix-quick-install-action which pairs well with
nix-community/cache-nix-action.

Should help with build times by reducing the number of packages needing
to be re-downloaded on each run.

Parameters are taken from https://github.com/nix-community/cache-nix-action
and may be tweaked later.

.github/workflows/nix-build.yml

@@ -6,8 +6,36 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: DeterminateSystems/nix-installer-action@main
+
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - name: Install Nix
+        uses: nixbuild/nix-quick-install-action@v31
+        with:
+          nix_conf: |
+            keep-env-derivations = true
+            keep-outputs = true
+
+      - name: Restore and save Nix store
+        uses: nix-community/cache-nix-action@v6
+        with:
+          # restore and save a cache using this key
+          primary-key: nix-${{ runner.os }}-${{ hashFiles('**/*.nix', '**/flake.lock') }}
+          # if there's no cache hit, restore a cache by this prefix
+          restore-prefixes-first-match: nix-${{ runner.os }}-
+          # collect garbage until the Nix store size (in bytes) is at most this number
+          # before trying to save a new cache
+          # 1G = 1073741824
+          gc-max-store-size-linux: 1G
+          # do purge caches
+          purge: true
+          # purge all versions of the cache
+          purge-prefixes: nix-${{ runner.os }}-
+          # created more than this number of seconds ago
+          purge-created: 0
+          # or, last accessed more than this number of seconds ago
+          # relative to the start of the `Post Restore and save Nix store` phase
+          purge-last-accessed: 0
+          # except any version with the key that is the same as the `primary-key`
+          purge-primary-key: never
 
       - name: Update inputs
         run: nix flake update