ci: Prevent ~/.stack/pantry cache from being empty

When building with an LTS version that has exactly the dependencies we
need (X11-1.9.2), stack doesn't need to download the Hackage index. If
GitHub Actions cache locking chooses this job as the one that writes the
cache, then the "stack-pantry-Linux" cache entry stays empty, possibly
forever.

Force Hackage index update to prevent this from happening.

Related: xmonad/xmonad-contrib@d1a4820

.github/workflows/stack.yml

@@ -66,6 +66,12 @@ jobs:
             stack-${{ runner.os }}-${{ matrix.resolver }}-${{ hashFiles('stack.yaml') }}-
             stack-${{ runner.os }}-${{ matrix.resolver }}-
 
+      - name: Update hackage index
+        # always update index to prevent the shared ~/.stack/pantry cache from being empty
+        run: |
+          set -ex
+          stack update
+
       - name: Build and test
         run: |
           set -ex