Clear stack cache and make cache-key more fine-grained

Attempt at fixing #137

.github/workflows/build-all-versions.yml

@@ -84,7 +84,7 @@ jobs:
       name: Cache ~/.stack
       with:
         path: ~/.stack
-        key: ${{ runner.os }}-${{ matrix.ghc }}-stack
+        key: ${{ runner.os }}-${{ matrix.ghc }}-stack--${{ hashFiles(format('stack-ghc{0}', matrix.ghc)) }}
 
     - name: Build
       run: |