Update build and test GitHub action to use nvmrc instead of node matrix

Author: voidrender

.github/workflows/push.yml

@@ -6,16 +6,15 @@ jobs:
     build:
         runs-on: ubuntu-latest
 
-        strategy:
-            matrix:
-                node-version: [8.x, 10.x, 12.x]
-
         steps:
             - uses: actions/checkout@v2
-            - name: Use Node.js ${{ matrix.node-version }}
+            - name: Read .nvmrc
+              run: echo "##[set-output name=NVMRC;]$(cat .nvmrc)"
+              id: nvm
+            - name: Use Node.js (.nvmrc)
               uses: actions/setup-node@v1
               with:
-                  node-version: ${{ matrix.node-version }}
+                  node-version: '${{ steps.nvm.outputs.NVMRC }}'
             - run: npm install
             - run: npm run build --if-present
             - run: npm test