Use cabal.project in cache invalidation

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.

(taken from https://github.com/clash-lang/clash-compiler/, including
commit detail message)

.github/workflows/ci.yaml

@@ -84,7 +84,7 @@ jobs:
           path: ${{ steps.setup-haskell.outputs.cabal-store }}
           key:
             ${{ runner.os }}-ghc-${{ matrix.ghc }}-${{ hashFiles('frozen',
-            'cabal.project.local') }}
+            'cabal.project', 'cabal.project.local') }}
           restore-keys: |
             ${{ runner.os }}-ghc-${{ matrix.ghc }}-