ci: Remove nix actions cache and prepopulate job

The nix actions cache currently leads to repeated "no space left on
devices" errors for jobs in CI.

The prepopulate job is useless without the nix actions cache, so it
will go away at the same time.

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

@@ -7,9 +7,6 @@ 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
@@ -17,12 +14,6 @@ runs:
     - uses: nixbuild/nix-quick-install-action@v26
       with:
         nix_version: '2.13.6'
-    - name: Restore and cache Nix store
-      uses: nix-community/cache-nix-action@v4.0.3
-      with:
-        key: cache-nix-${{ runner.os }}-${{ inputs.cache }}-${{ hashFiles('**/*.nix', 'nix/**/*.patch') }}
-        restore-keys: |
-          cache-nix-${{ runner.os }}-${{ inputs.cache }}-
     - uses: cachix/cachix-action@v14
       with:
         name: postgrest