cmd/relui: ignore x/vuln when tagging x/ repos

x/vuln will soon start tagging releases, but does not want automatic
tagging yet, only manual tags. However, as soon as the first tag is
created then the repo will become eligible for automatic tagging.

Add an explicit ignore for x/vuln to the task to avoid eligibility.

This isn't ideal because the single tag task also uses the ignore list,
so tags must be created manually, but this unblocks the process.

For golang/go#59686.

Change-Id: Iac190e1da5edd328aacdb001b7ef6acfefafdb98
Reviewed-on: https://go-review.googlesource.com/c/build/+/485978
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>

cmd/relui/main.go

@@ -207,6 +207,7 @@ func main() {
 	for p, r := range repos.ByGerritProject {
 		ignoreProjects[p] = !r.ShowOnDashboard()
 	}
+	ignoreProjects["vuln"] = true // x/vuln only has manual tagging for now. See issue 59686.
 	tagTasks := &task.TagXReposTasks{
 		IgnoreProjects:   ignoreProjects,
 		Gerrit:           gerritClient,