Use step outputs for cache key

Author: jneira

.github/workflows/flags.yml

@@ -86,7 +86,7 @@ jobs:
           echo "INDEX_STATE=$INDEX_STATE1" >> $GITHUB_ENV
 
       - name: Form the package list ('cabal.project.freeze')
-        continue-on-error: true
+        id: compute-cache-key
         run: |
           cabal v2-freeze && \
           echo "" && \
@@ -95,6 +95,7 @@ jobs:
           cat 'cabal.project.freeze' && \
           echo "" || \
           echo 'WARNING: Could not produce the `freeze`.'
+          echo ::set-output name=value::${{ hashFiles('cabal.project.freeze') }}
 
       - name: Hackage sources cache
         uses: actions/cache@v2
@@ -112,7 +113,7 @@ jobs:
           cache-name: compiled-deps
         with:
           path: ${{ steps.HaskEnvSetup.outputs.cabal-store }}
-          key:  ${{ env.cache-name }}-${{ runner.os }}-${{ matrix.ghc }}-${{ env.INDEX_STATE }}-${{ hashFiles('cabal.project.freeze') }}
+          key:  ${{ env.cache-name }}-${{ runner.os }}-${{ matrix.ghc }}-${{ env.INDEX_STATE }}-${{ steps.compute-cache-key.outputs.value  }}
           restore-keys: |
                 ${{ env.cache-name }}-${{ runner.os }}-${{ matrix.ghc }}-${{ env.INDEX_STATE }}-
                 ${{ env.cache-name }}-${{ runner.os }}-${{ matrix.ghc }}-