ci: Refresh caches once a month

GitHub Actions writes caches on the first miss and then never updates
them again. If the CI is used frequently, the caches never expire and
as they get old, become less useful.

To avoid this, force refreshing the caches once a month.

.github/workflows/stack.yml

@@ -60,11 +60,20 @@ jobs:
           sudo apt install -y ghc-${{ matrix.ghc }}
           echo /opt/ghc/${{ matrix.ghc }}/bin >> $GITHUB_PATH
 
+      - name: Refresh caches once a month
+        id: cache-date
+        # GHA writes caches on the first miss and then never updates them again;
+        # force updating the cache at least once a month
+        run: |
+          echo "::set-output name=date::$(date +%Y-%m)"
+
       - name: Cache Haskell package metadata
         uses: actions/cache@v2
         with:
           path: ~/.stack/pantry
-          key: stack-pantry-${{ runner.os }}
+          key: stack-pantry-${{ runner.os }}-${{ steps.cache-date.outputs.date }}
+          restore-keys: |
+            stack-pantry-${{ runner.os }}-
 
       - name: Cache Haskell dependencies
         uses: actions/cache@v2
@@ -72,9 +81,10 @@ jobs:
           path: |
             ~/.stack/*
             !~/.stack/pantry
-          key: stack-${{ runner.os }}-${{ matrix.resolver }}-${{ hashFiles(matrix.yaml) }}-${{ hashFiles('*.cabal') }}
+          key: stack-${{ runner.os }}-${{ matrix.resolver }}-${{ steps.cache-date.outputs.date }}-${{ hashFiles(matrix.yaml) }}-${{ hashFiles('*.cabal') }}
           restore-keys: |
-            stack-${{ runner.os }}-${{ matrix.resolver }}-${{ hashFiles(matrix.yaml) }}-
+            stack-${{ runner.os }}-${{ matrix.resolver }}-${{ steps.cache-date.outputs.date }}-${{ hashFiles(matrix.yaml) }}-
+            stack-${{ runner.os }}-${{ matrix.resolver }}-${{ steps.cache-date.outputs.date }}-
             stack-${{ runner.os }}-${{ matrix.resolver }}-
 
       - name: Update hackage index