Update and rename blank.yml to build.yml

Author: kewlbear

.github/workflows/build.yml

@@ -0,0 +1,13 @@
+name: CI
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: macOS-latest
+    
+    steps:
+    - uses: actions/checkout@v1
+    - name: build
+      run: ./build-ffmpeg.sh 2>.stderr