ci/pr-deploy: use git ref in cache key, but restore optimistically

This will create different caches for each PR, and restore the most recent cache (that still matches gatsby config files) if there is not a cache entry for a particular PR.
This allows to keep the cache fresh, as the cache is not re-uploaded if `key` hits, but _is_ reuploaded if restore-keys. As the most recent key is fetched from `restore-keys`, this ensures we keep refreshing the cache instead of using always the same one that could have been generated a long time ago.

Author: nadiamoe

.github/workflows/pr-deploy.yml

@@ -110,7 +110,9 @@ jobs:
             public
           # Cache will not be used by Gatsby if the following files change:
           # https://www.gatsbyjs.com/docs/build-caching/
-          key: pr-gatsby-${{ hashFiles('package.json', 'gatsby-config.js', 'gatsby-node.js') }}
+          key:  pr-gatsby-${{ hashFiles('package.json', 'gatsby-config.js', 'gatsby-node.js') }}-${{ github.ref }}
+          restore-keys: |
+            pr-gatsby-${{ hashFiles('package.json', 'gatsby-config.js', 'gatsby-node.js') }}-
 
       - name: Build
         run: npm run build