ci: cache Nix store with cache-nix-action (#2992)

* ci: try nix-community/cache-nix-action

* ci: add cache-id param to setup-nix action

* ci: tidy up cache keys for non-nix jobs

* ci: merge-nix-caches-linux job

* ci: merge caches other way around

* ci: reduce number of caches

Should prevent disk overflow

* ci: change cache id prefix for merge

* ci: comment out cache merging job

* ci: revert cache id prefix

* ci: setup-nix: use latest cache-nix-action

Among others, makes action logs look more tidy (see https://github.com/nix-community/cache-nix-action/commit/17d19d3d8be918757589635bc1a6830be0b129d2)

* ci: use test-pg cache key for loadtest

.github/actions/setup-nix/action.yaml

@@ -7,13 +7,23 @@ inputs:
     description: Token to pass to cachix
   tools:
     description: Tools to install with nix-env -iA <tools>
+  cache-id:
+    description: Cache id to use for cache-nix-action
+    default: "default"
 
 runs:
   using: composite
   steps:
-    - uses: cachix/install-nix-action@v23
+    - uses: nixbuild/nix-quick-install-action@v26
       with:
-        install_url: https://releases.nixos.org/nix/nix-2.13.3/install
+        nix_version: '2.13.6'
+    - name: Restore and cache Nix store
+      uses: nix-community/cache-nix-action@v4.0.3
+      with:
+        key: cache-nix-${{ runner.os }}-id-${{ inputs.cache-id }}-${{ hashFiles('nix/**/*.nix') }}
+        restore-keys: |
+          cache-nix-${{ runner.os }}-common-
+        restore-key-hit: true
     - uses: cachix/cachix-action@v12
       with:
         name: postgrest