Allow different workflows/PRs to share cached Julia environment

.github/workflows/preview.yml

@@ -31,6 +31,7 @@ jobs:
       - name: Load Julia packages from cache
         uses: julia-actions/cache@v2
         with:
+          cache-name: julia-cache;${{ hashFiles('**/Manifest.toml') }}
           delete-old-caches: false
 
       # Note: needs resolve() to fix #518