ci: install subversion before bootstrap (svn no longer preinstalled)

ubuntu-latest dropped svn from the default image. install-wp-tests.sh
still uses svn to fetch the WP test suite from develop.svn.wordpress.org.
Add a step to apt-install subversion before the bootstrap runs.

Agent-Session: cowork-pr-theme-tests-20260414

.github/workflows/ci.yml

@@ -75,6 +75,10 @@ jobs:
         options: --health-cmd="mysqladmin ping" --health-interval=10s --health-timeout=5s --health-retries=5
     steps:
       - uses: actions/checkout@v4
+      - name: Install subversion
+        # ubuntu-latest no longer ships svn preinstalled. The WP test suite
+        # bootstrap script checks out develop.svn.wordpress.org via svn.
+        run: sudo apt-get update -qq && sudo apt-get install -y --no-install-recommends subversion
       - name: Setup PHP ${{ matrix.php }}
         uses: shivammathur/setup-php@v2
         with: