feat(besadii): Propagate Gerrit change ID & patchset to Buildkite

I previously implemented this in a CL that ended up being abandoned,
but it turns out we need it for the hook setup, anyways.

These environment variables become available during the build and,
crucially, to the post-build hooks.

Change-Id: Id6c1657947995e8bae1fa7b76184dd8be4c01525
Reviewed-on: https://cl.tvl.fyi/c/depot/+/739
Reviewed-by: Kane York <rikingcoding@gmail.com>

ops/besadii/main.go2

@@ -47,17 +47,21 @@ type Author struct {
 // Build is the representation of a Buildkite build as described on
 // https://buildkite.com/docs/apis/rest-api/builds#create-a-build
 type Build struct {
-	Commit  string `json:"commit"`
-	Branch  string `json:"branch"`
-	Message string `json:"message"`
-	Author  Author `json:"author"`
+	Commit  string            `json:"commit"`
+	Branch  string            `json:"branch"`
+	Message string            `json:"message"`
+	Author  Author            `json:"author"`
+	Env     map[string]string `json:"env"`
 }
 
 // Trigger a build of a given branch & commit on Buildkite
 func triggerBuild(log *syslog.Writer, token string, update *refUpdated) error {
 	var message string
+	env := make(map[string]string)
 
 	if update.changeId != nil && update.patchset != nil {
+		env["GERRIT_CHANGE_ID"] = *update.changeId
+		env["GERRIT_PATCHSET"] = *update.patchset
 		message = fmt.Sprintf(":llama: depot @ https://cl.tvl.fyi/c/depot/+/%s/%s", *update.changeId, *update.patchset)
 	} else {
 		message = fmt.Sprintf(":llama: depot @ %s", update.commit)
@@ -67,6 +71,7 @@ func triggerBuild(log *syslog.Writer, token string, update *refUpdated) error {
 		Commit:  update.commit,
 		Branch:  update.ref,
 		Message: message,
+		Env:     env,
 		Author: Author{
 			Name:  update.submitter,
 			Email: update.email,