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.yaml

@@ -1,15 +1,33 @@
 name: Build xdph (Nix)
-
-on: [push, pull_request, workflow_dispatch]
+'on':
+  - push
+  - pull_request
+  - workflow_dispatch
 jobs:
   nix:
-    name: "Build"
+    name: Build
     runs-on: ubuntu-latest
     steps:
-    - name: Clone repository
-      uses: actions/checkout@v3
-
-    - uses: cachix/install-nix-action@v27
-
-    - name: Build xdg-desktop-portal-hyprland
-      run: nix build --print-build-logs --extra-substituters "https://hyprland.cachix.org"
+      - name: Clone repository
+        uses: actions/checkout@v3
+      - 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:
+          primary-key: 'nix-${{ runner.os }}-${{ hashFiles(''**/*.nix'', ''**/flake.lock'') }}'
+          restore-prefixes-first-match: 'nix-${{ runner.os }}-'
+          gc-max-store-size-linux: 1G
+          purge: true
+          purge-prefixes: 'nix-${{ runner.os }}-'
+          purge-created: 0
+          purge-last-accessed: 0
+          purge-primary-key: never
+      - name: Build xdg-desktop-portal-hyprland
+        run: >-
+          nix build --print-build-logs --extra-substituters
+          "https://hyprland.cachix.org"