Create makefile.yml

Automate code formatting and environment file exporting on every push and pull request.

.github/workflows/makefile.yml

@@ -0,0 +1,21 @@
+name: Makefile CI
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Format
+      run: make fmt
+
+    - name: Export environment
+      run: make exportenv