cmd/relui: actually fix publishing

I got the arguments to TrimPrefix backward, but more importantly,
u.Query() returns a temporary copy.

For golang/go#51797.

Change-Id: I37a9df2adf47f1d72236d02761989d62e9991fc7
Reviewed-on: https://go-review.googlesource.com/c/build/+/411064
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Heschi Kreinick <heschi@google.com>
diff --git a/cmd/relui/main.go b/cmd/relui/main.go
index 67e9f0f..2245ecd 100644
--- a/cmd/relui/main.go
+++ b/cmd/relui/main.go
@@ -183,8 +183,10 @@
 	if err != nil {
 		return fmt.Errorf("invalid website upload URL %q: %v", *websiteUploadURL, err)
 	}
-	u.Query().Set("user", strings.TrimPrefix("user-", auth.Username))
-	u.Query().Set("key", auth.Password)
+	q := u.Query()
+	q.Set("user", strings.TrimPrefix(auth.Username, "user-"))
+	q.Set("key", auth.Password)
+	u.RawQuery = q.Encode()
 	resp, err := http.Post(u.String(), "application/json", bytes.NewReader(req))
 	if err != nil {
 		return err