dashboard: add SlowBot alias for freebsd-arm64-dmgk

This should fix TestSlowBotAliases on tip.

Updates golang/go#24715
Updates golang/go#34501

Change-Id: I80f99a5c24ef0860717605816a753fe6625aff17
Reviewed-on: https://go-review.googlesource.com/c/build/+/203421
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: tklauser

dashboard/builders.go

@@ -58,6 +58,7 @@ var slowBotAliases = map[string]string{
 	"freebsd-386":    "freebsd-386-12_0",
 	"freebsd-amd64":  "freebsd-amd64-12_0",
 	"freebsd-arm":    "freebsd-arm-paulzhol",
+	"freebsd-arm64":  "freebsd-arm64-dmgk",
 	"illumos":        "illumos-amd64",
 	"ios":            "darwin-arm64-corellium",
 	"js":             "js-wasm",