Fix build cache key

.github/workflows/build.yml

@@ -49,7 +49,7 @@ jobs:
           ### If jekyll_src is set, the action would automatically choose the Gemfile in jekyll_src.
           ### In that case this input may not be needed as well.
           #
-          # key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
+          key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
           # restore-keys: ${{ runner.os }}-gems-
           ### In cases where you want to specify the cache key, enable the above 2 inputs
           ### Follows the format here https://github.com/actions/cache