dashboard: set GO111MODULE=on for x/perf

CL 191378 removed some +build appengine tags from
./analysis/appengine, so those packages now require
GO111MODULE=on in order to resolve the dependency
on google.golang.org/appengine.

Fixes golang/go#33952

Change-Id: I20c9ba3f42d488265726fda9b1c790e48a1433b3
Reviewed-on: https://go-review.googlesource.com/c/build/+/192333
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index a0a89d0..863cad4 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -780,7 +780,7 @@
 		if !c.OutboundNetworkAllowed() {
 			env = append(env, "GOPROXY=off")
 		}
-	case "oauth2", "build", "website":
+	case "oauth2", "build", "perf", "website":
 		env = append(env, "GO111MODULE=on")
 	}
 	return