JpegDecoderTests.Decode_IsCancellable is flaky on CI

[antonfirsov]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

It's good to have a test for this IMO, however the sporadic CI failures are annoying.
Idea: add a retry loop around it, observe cancellation at least once from 5 runs.