Install yarn in the on-pr workflow

.gitea/workflows/on-pr.yaml

@@ -11,6 +11,10 @@ jobs:
         node-version: [20.x]
     steps:
       - uses: actions/checkout@v3
+      - name: Download yarn
+        run: |
+          curl -fsSL -o /usr/local/bin/yarn https://github.com/yarnpkg/yarn/releases/download/v1.22.21/yarn-1.22.21.js
+          chmod +x /usr/local/bin/yarn
       - name: Use Node.js ${{ matrix.node-version }}
         uses: actions/setup-node@v3
         with: