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 <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Heschi Kreinick <[email protected]>

Author: heschi

cmd/relui/main.go

@@ -183,8 +183,10 @@ func publishFile(uploadURL string, auth buildlet.UserPass, f *relui.WebsiteFile)
 	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