only run setup ruby on arm

.gitea/workflows/test-pr.yml

@@ -12,6 +12,11 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v3
+      - uses: ruby/setup-ruby@v1
+        if: matrix.os == 'arm-ubuntu-latest'
+        with:
+          ruby-version: '3.0' # Not needed with a .ruby-version file
+          bundler-cache: true # runs 'bundle install' and caches installed gems automatically
       - name: install tools
         run: |
           apt update -y