vendor/cmd/go: fix TestConvertLegacyConfig

"vgo build" caches to $GOPATH, which defaults to $HOME/go. Therefore,
tests of a "build" command must set at least one of {$GOPATH, $HOME}.

Updates golang/go#25501.

Change-Id: I798a0b180c3d8c4850e9a54e303bbcef1bab5faf
Reviewed-on: https://go-review.googlesource.com/117235
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/vendor/cmd/go/vgo_test.go b/vendor/cmd/go/vgo_test.go
index 694c150..d742333 100644
--- a/vendor/cmd/go/vgo_test.go
+++ b/vendor/cmd/go/vgo_test.go
@@ -289,6 +289,7 @@
 	defer tg.cleanup()
 	tg.makeTempdir()
 
+	tg.setenv("HOME", tg.path("."))
 	tg.must(os.MkdirAll(tg.path("x"), 0777))
 	tg.must(ioutil.WriteFile(tg.path("x/x.go"), []byte(`package x; import _ "github.com/gorilla/mux"`), 0666))
 	gomod := []byte(`