Cache nix store for CI actions

.github/workflows/nix-build.yml

@@ -8,6 +8,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: cachix/install-nix-action@v8
+      - name: Cache nix store
+        uses: actions/cache@v1
+        with:
+          path: /nix
+          key: ${{ runner.os }}-nix-store
       - uses: cachix/cachix-action@v5
         with:
           name: hitsofcode