replace travis (part 1): add empty github workflow (census-instrumentation#979)

Author: aabmass

.github/workflows/build.yml

@@ -0,0 +1,14 @@
+name: Build
+
+on:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build and test
+        run: echo "Hello world!"