Add simple github actions

Makes it easy for anyone to run tests on github, doesn't preclude us
using prow also.

.github/workflows/main.yml

@@ -0,0 +1,48 @@
+---
+name: CI
+
+'on':
+  - push
+  - pull_request
+
+env:
+  GOPROXY: https://proxy.golang.org
+  GOPATH: ${{ github.workspace }}/go
+
+permissions:
+  contents: read
+
+jobs:
+  build-linux-amd64:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up go
+        uses: actions/setup-go@v4
+        with:
+          go-version: '1.20.6'
+
+      - uses: actions/checkout@v3
+        with:
+          path: git-sync
+
+      - name: make all-build
+        working-directory: git-sync
+        run: |
+          make all-build
+
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up go
+        uses: actions/setup-go@v4
+        with:
+          go-version: '1.20.6'
+
+      - uses: actions/checkout@v3
+        with:
+          path: git-sync
+
+      - name: make test
+        working-directory: git-sync
+        run: |
+          make test