DEV: Cache plugin gems when running plugin test jobs in Github CI (#24535)

Why this change?

Plugin gems for official plugins are being installed over and over again
each time we run RSpec and QUnit tests for plugins. In particular, the
rugged gem installed by the discourse-code-review plugin takes
approximately 50-60 seconds to install because it is compiling libgit2.

.github/workflows/tests.yml

@@ -117,6 +117,12 @@ jobs:
         if: matrix.target == 'plugins'
         run: bin/rake plugin:pull_compatible_all
 
+      - name: Plugin gems cache
+        uses: actions/cache@v3
+        with:
+          path: plugins/*/gems
+          key: ${{ runner.os }}-plugin-gems-${{matrix.ruby}}-${{ hashFiles('plugins/*/plugin.rb') }}
+
       - name: Checkout official themes
         if: matrix.target == 'themes' && matrix.build_type == 'system'
         run: bin/rake themes:clone_all_official