cmd/coordinator: warn about known linux-arm SlowBot issue

The current linux-arm builder is known to have trouble when used as
a SlowBot. Start warning about it when the builder is requested via
the TRY= SlowBot UI.

I've considered also removing or disabling the "arm" SlowBot alias,
but that would make it easier to miss that there's an issue, since
SlowBots don't warn about unknown builders:

	If you specify an unknown TRY= token, it'll just ignore it
	and won't report an error.

We can consider making further changes as this situation evolves.
The goal here is to start notifying about a known problem sooner.

For golang/go#35628.
For golang/go#40872.

Change-Id: Ibc1205720c44ec4823c632c04fc2f887368258c1
Reviewed-on: https://go-review.googlesource.com/c/build/+/249420
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

cmd/coordinator/coordinator.go

@@ -1383,6 +1383,14 @@ func (ts *trySet) notifyStarting() {
 	}
 	msg := name + " beginning. Status page: https://farmer.golang.org/try?commit=" + ts.Commit[:8]
 
+	// If any of the requested SlowBot builders
+	// have a known issue, give users a warning.
+	for _, b := range ts.slowBots {
+		if b.KnownIssue != 0 {
+			msg += fmt.Sprintf("\nNote that builder %s has a known issue golang.org/issue/%d.", b.Name, b.KnownIssue)
+		}
+	}
+
 	gerritClient := pool.NewGCEConfiguration().GerritClient()
 	ctx := context.Background()
 	if ci, err := gerritClient.GetChangeDetail(ctx, ts.ChangeTriple()); err == nil {

dashboard/builders.go

@@ -1760,7 +1760,8 @@ func init() {
 			GOARM:              "7",
 			AlwaysCrossCompile: false,
 		},
-		tryBot:            nil, // Issue 22748, Issue 22749
+		tryBot:            nil, // Issues #22748, #22749, #35628, #40872.
+		KnownIssue:        35628,
 		FlakyNet:          true,
 		numTestHelpers:    2,
 		numTryTestHelpers: 7,