add pr workflows

.github/workflows/pxt-buildpr.yml

@@ -0,0 +1,31 @@
+name: pxt-buildpr
+
+on: [pull-request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [8.x]
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
+      - name: npm install
+        run: |
+          sudo apt-get install xvfb
+          sudo npm install -g pxt
+          npm install
+      - name: pxt ci
+        run: |
+          pxt ci
+        env:
+          CHROME_BIN: chromium-browser
+          DISPLAY: :99.0
+          CI: true

.github/workflows/pxt-buildtarget.yml

@@ -1,6 +1,8 @@
 name: pxt-buildtarget
 
-on: [push]
+on:
+  push:
+    branches: master
 
 jobs:
   build: