Use cabal.project in cache key calculation

Cabal's freeze file doesn't include version hashes, just version
numbers. This means that updates to dependencies specified in
`cabal.project`'s `source-repository-package` field don't get taken into
account properly.

This issue doesn't affect Stack, as it only relies on version hashes in
the first place.

.github/workflows/ci.yml

@@ -165,7 +165,7 @@ jobs:
             dist-newstyle
             ~/.cabal/store
 
-          key: ${{ matrix.ghc }}-${{ hashFiles('frozen') }}
+          key: ${{ matrix.ghc }}-${{ hashFiles('frozen', 'cabal.project') }}
           restore-keys: ${{ matrix.ghc }}-
 
       - name: Build Clash