Fix crash in ImageDownloader

[kean]

This is a potential fix for this crash in iOS 18 betas. I assume, based on the new signature of the withTaskCancellationHandler method that the crash has something to do with isolation, so I moved it to a context with no isolation whatsoever, so hopefully it fixes it.

To test:

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 25.4.1. This milestone is due in less than 4 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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[kean]

This is now called with no isolation, so hopefully it will skip the part of the runtime that crashes on iOS 18 betas.