dashboard: disable pkgsite on plan9 builders

Pkgsite depends on github.com/lib/pq for its database driver. pq is
broken on plan9. I've sent a pull request to fix them but it seems
like it will be a while until it's looked at. Disable the builders for
now.

For golang/go#61209

Change-Id: I1fba995662cc0b1756ac00b9de11d264412f2f79
Reviewed-on: https://go-review.googlesource.com/c/build/+/508864
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

dashboard/builders.go

@@ -3527,6 +3527,9 @@ func plan9Default(repo, branch, goBranch string) bool {
 	case "vulndb", "vuln":
 		// vulncheck can't read plan9 binaries.
 		return false
+	case "pkgsite":
+		// pkgsite has a dependency (github.com/lib/pq) that is broken on Plan 9.
+		return false
 	default:
 		return onlyMasterDefault(repo, branch, goBranch)
 	}