DEV: Pull compatible themes in tests workflow (#27093)

This commit adds a step in our tests workflow on Github actions to update the themes to
use the compatible version when not running aginast the `main` branch.
This is to ensure that we are not running
the tests for themes against an incompatible version of Discourse.

.github/workflows/tests.yml

@@ -145,6 +145,10 @@ jobs:
         if: matrix.target == 'themes' && matrix.build_type == 'system'
         run: bin/rake themes:clone_all_official
 
+      - name: Pull compatible versions of themes
+        if: matrix.target == 'themes'
+        run: bin/rake themes:pull_compatible_all
+
       - name: Add hosts to /etc/hosts, otherwise Chromium cannot reach minio
         if: matrix.build_type == 'system' && matrix.target == 'core'
         run: |