cmd/go: add build flags for get cmd

In CL 116756, we add "-x" flag to show git commands, but we not
define it in get cmd, add all build flags for get cmd.

Fixes golang/go#25918

Change-Id: I67ce57721f44c0a84f7dc888034595f3044eb51d
Reviewed-on: https://go-review.googlesource.com/119616
Run-TryBot: Baokun Lee <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: oiooj

vendor/cmd/go/main.go

@@ -84,6 +84,7 @@ func Main() {
 
 	if vgo.MustBeVgo {
 		// If running as vgo or with -vgo, change get now to change help message.
+		work.AddBuildFlags(vgo.CmdGet)
 		*get.CmdGet = *vgo.CmdGet
 	}
 
@@ -143,6 +144,7 @@ func Main() {
 		vgo.Init()
 		if !vgo.MustBeVgo && vgo.Enabled() {
 			// Didn't do this above, so do it now.
+			work.AddBuildFlags(vgo.CmdGet)
 			*get.CmdGet = *vgo.CmdGet
 		}
 	}