dashboard: set GO111MODULE=auto explicitly for most subrepos

For subrepos that haven't been explicitly opted-into using
GO111MODULE=on by now, set the default of GO111MODULE=auto.
This is needed now because not all subrepos are ready to be
built in GO111MODULE=on mode, but the tip of Go has recently
changed (CL 162698) the default value of GO111MODULE from
auto to on.

DO NOT SUBMIT: this is a WIP

Updates golang/go#30752

Change-Id: I0616522656f4e0e78f5a859efb489b956f21b6fb
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 6615902..e13f9de 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -749,8 +749,12 @@
 		env = append(env, "GO_BUILDER_SET_GOPROXY=coordinator")
 	}
 	switch repo {
+	case "go":
+		// Don't do anything for main repo.
 	case "oauth2", "build", "website":
 		env = append(env, "GO111MODULE=on")
+	default:
+		env = append(env, "GO111MODULE=auto")
 	}
 	return
 }