Use Node 15 with bundled npm 7, see actions/setup-node#213

Author: josephfrazier

.github/workflows/build.yml

@@ -11,7 +11,8 @@ jobs:
     - uses: actions/checkout@v2
     - name: Use Node.js
       uses: actions/setup-node@v1
-    - run: npm install -g npm@7
+      with:
+        node-version: '15'
     - run: npm install
     - run: npm test
     - shell: bash