dashboard: add known issue for openbsd-arm64-jsing

For golang/go#52653.
Updates golang/go#52703.

Change-Id: I0873c19865d17b387bfe7f7e2d21b3bb343d0855
Reviewed-on: https://go-review.googlesource.com/c/build/+/408703
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

dashboard/builders.go

@@ -2132,6 +2132,7 @@ func init() {
 			// The machine is slow.
 			"GO_TEST_TIMEOUT_SCALE=5",
 		},
+		KnownIssues: []int{52703},
 	})
 	addBuilder(BuildConfig{
 		Name:         "openbsd-mips64-jsing",