add yarn

.github/workflows/build_jlext.yml

@@ -16,6 +16,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       
+      - name: Install yarn
+        run: |-
+          curl -fsSL --create-dirs -o $HOME/bin/yarn \
+          https://github.com/yarnpkg/yarn/releases/download/v1.22.22/yarn-1.22.22.js
+          chmod +x $HOME/bin/yarn
+          echo "$HOME/bin" >> $GITHUB_PATH
+
       - name: Base Setup
         uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1