dashboard: add privateGoProxy setting for linux-loong64-3a5000

Because these builders are behind the firewall, they may not access to
proxy.golang.org, but it has a private Go module proxy.

For golang/go#46229.

Change-Id: Ib125cdaafca465f85531750cb9f5eb36d54f02de
Reviewed-on: https://go-review.googlesource.com/c/build/+/408434
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2547,6 +2547,7 @@ func init() {
 		SkipSnapshot:   true,
 		distTestAdjust: loong64DistTestPolicy,
 		buildsRepo:     loong64BuildsRepoPolicy,
+		privateGoProxy: true, // this builder is behind firewall
 		env: []string{
 			"GOARCH=loong64",
 			"GOHOSTARCH=loong64",