ci: Split cabal and stack work caches from regular cache

This is a first step to split up the cabal and stack caches in separate
pieces. Here we split the work folder, which just contains the
postgrest-specific build artifacts, into a separate cache.

More fine-grained caching should give us better cache hits and much
fewer upload size in the regular case, improving CI performance.

Since the work file caches are very small (about 30-40 MB) they are
cached for PRs, too. This will allow the majority of PRs, which only
change source code files, but no dependencies, to still have cached
their build files for additional commits.

.github/actions/cache-on-main/action.yaml

@@ -1,29 +1,33 @@
 name: Cache on main
 
-description: Stores caches on main only, but restores them on all branches.
+description: Stores caches on main and release branches only, but restores them on all branches.
 
 inputs:
   path:
     description: Path(s) to cache
     required: true
+  save-prs:
+    description: Whether to additionally store the cache in a pull request, too. Should only be used for very small caches.
+    default: false
   prefix:
     description: Cache key prefix to be used in both primary key and restore-keys.
     required: true
   suffix:
     description: Cache key suffix to be used only in primary key.
+    required: true
 
 runs:
   using: composite
   steps:
     - uses: actions/cache@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
-      if: ${{ startsWith(github.ref, 'refs/heads/') }}
+      if: ${{ startsWith(github.ref, 'refs/heads/') || (inputs.save-prs && github.head_ref) }}
       with:
         path: ${{ inputs.path }}
         key: ${{ runner.os }}-${{ inputs.prefix }}-${{ inputs.suffix }}
         restore-keys: |
           ${{ runner.os }}-${{ inputs.prefix }}-
     - uses: actions/cache/restore@0c45773b623bea8c8e75f6c82b208c3cf94ea4f9 # v4.0.2
-      if: ${{ !startsWith(github.ref, 'refs/heads/') }}
+      if: ${{ !startsWith(github.ref, 'refs/heads/') && !(inputs.save-prs && github.head_ref)  }}
       with:
         path: ${{ inputs.path }}
         key: ${{ runner.os }}-${{ inputs.prefix }}-${{ inputs.suffix }}