internal/relui: write to "ready" file, fix signing arg
internal/relui: write to "ready" file, fix signing arg

Simply opening and closing a file isn't enough to create it.

Also, don't use path.Join on URLs: it converts gs://foo to gs:/foo which
is a very different URL.

For golang/go#51797.

Change-Id: Ic681e151f2a2258f93e055564589d4440358e954
Reviewed-on: https://go-review.googlesource.com/c/build/+/412390
Auto-Submit: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build