dashboard: only build tip on netbsd-arm64-bsiegert builder

The build fails on 1.15 and earlier.

Updates golang/go#30824

Change-Id: I1f62d37cc05dde988cbb7e5124bd97916b38bcb3
Reviewed-on: https://go-review.googlesource.com/c/build/+/257938
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

dashboard/builders.go

@@ -1953,8 +1953,11 @@ func init() {
 		},
 	})
 	addBuilder(BuildConfig{
-		Name:           "netbsd-arm64-bsiegert",
+		Name:     "netbsd-arm64-bsiegert",
-		HostType:       "host-netbsd-arm64-bsiegert",
+		HostType: "host-netbsd-arm64-bsiegert",
+		buildsRepo: func(repo, branch, goBranch string) bool {
+			return atLeastGo1(goBranch, 16) && buildRepoByDefault(repo)
+		},
 		distTestAdjust: noTestDirAndNoReboot,
 		tryBot:         nil,
 		env: []string{