dashboard: add known issue for plan9-arm

For golang/go#52653.
Updates golang/go#49338.

Change-Id: Ib23be56bd79bce8b3168cb40064916a2a91dbb88
Reviewed-on: https://go-review.googlesource.com/c/build/+/408697
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Alex Rakoczy <alex@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

dashboard/builders.go

@@ -2697,6 +2697,7 @@ func init() {
 		HostType:       "host-plan9-arm-0intro",
 		distTestAdjust: noTestDirAndNoReboot,
 		buildsRepo:     plan9Default,
+		KnownIssues:    []int{49338},
 	})
 	addBuilder(BuildConfig{
 		Name:     "plan9-amd64-0intro",