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/workflows/loadtest.yaml

@@ -23,6 +23,7 @@ jobs:
         uses: ./.github/actions/setup-nix
         with:
           tools: loadtest
+          cache-id: test-pg
       - uses: actions-ecosystem/action-get-latest-tag@v1
         id: get-latest-tag
         with:
@@ -54,6 +55,7 @@ jobs:
         uses: ./.github/actions/setup-nix
         with:
           tools: loadtest
+          cache-id: test-pg
       - name: Run loadtest
         run: |
           postgrest-loadtest-against ${{ steps.get-latest-tag.outputs.tag }}