ci: use node.js version from matrix (#539)

* ci: use node.js version from matrix

* ci: include node.js v15

Author: dnalborczyk

.github/workflows/pull_request.yml

@@ -21,10 +21,12 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [10.x, 12.x, 14.x]
+        node-version: [10.x, 12.x, 14.x, 15.x]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
       - run: npm ci
       - run: npm run test:coverage
   typecheck: