Specify cache directory so GH Actions can locate gh-pages cache folder (#2542)

Author: EltonGohJH

packages/core/src/Site/index.ts

@@ -1405,6 +1405,8 @@ export class Site {
           email: '[email protected]',
         };
       } else if (process.env.GITHUB_ACTIONS) {
+        // Set cache folder to a location Github Actions can find.
+        process.env.CACHE_DIR = path.join(process.env.GITHUB_WORKSPACE || '', 'gh-pages', '.cache');
         repoSlug = Site.extractRepoSlug(options.repo, process.env.GITHUB_REPOSITORY);
 
         options.user = {