.github/workflows: Introduce a sanity check that runs static linting/style checks

Introduce the ci/sanity GH workflow:
- Runs `make vendor`
- Runs `make lint`
- Ensures that both of those targets don't produce a diff when running
  `make diff`

Author: timflannagan

.github/workflows/sanity.yaml

@@ -0,0 +1,23 @@
+name: ci
+
+on:
+  push:
+    branches:
+      - '**'
+  pull_request:
+    paths:
+      - '**'
+      - '!doc/**'
+
+jobs:
+  sanity:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-go@v2
+        with:
+          go-version: '~1.16'
+      - name: Install goimports
+        run: go install golang.org/x/tools/cmd/goimports@latest
+      - name: Run sanity checks
+        run: make vendor && make lint && make diff