Add CI step verifying go fmt

.buildkite/pipeline.yml

@@ -1,4 +1,12 @@
 steps:
+  - name: ":go: go fmt"
+    key: test-go-fmt
+    command: ".buildkite/steps/test-go-fmt.sh"
+    plugins:
+      - docker#v3.1.0:
+          image: "golang:1.20"
+          workdir: /go/src/github.com/buildkite/buildkite-agent-scaler
+
   - name: ":golang:"
     command: ".buildkite/steps/tests.sh"
     plugins:

.buildkite/steps/test-go-fmt.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+if [[ $(gofmt -l ./ | head -c 1 | wc -c) != 0 ]]; then
+  echo "The following files haven't been formatted with \`go fmt\`:"
+  gofmt -l ./
+  echo
+  echo "Fix this by running \`go fmt ./...\` locally, and committing the result."
+  exit 1
+fi
+
+echo "Everything is formatted! 🎉"