internal/relui/sign: skip flaky test TestUpdateSigningStatusError
internal/relui/sign: skip flaky test TestUpdateSigningStatusError

This skips the
TestUpdateSigningStatusError/non-existant_signing_request test.

Updates golang/go#54654

Change-Id: I0db94e95cdd6a6ad25696ea7029fbf93f9092d66
Reviewed-on: https://go-review.googlesource.com/c/build/+/425359
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Auto-Submit: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Run-TryBot: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

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