chore(deps): update flatpak/flatpak-github-actions action to v6.6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
flatpak/flatpak-github-actions	action	minor	v6.3 -> v6.6

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Release Notes

flatpak/flatpak-github-actions (flatpak/flatpak-github-actions)

v6.6

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• Specify full URL policy when mirroring screenshots
• Fix restore cache keys to actually include arch

v6.5

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• Recommend using images from Flathub instead of building our own. They have been failing to build for a long time and as Flathub is the distributor of the various SDKs, it just makes more sense for them to provide the developer-friendly containers/images to be used on CI as well.
• Update path to the mirrored screenshots
• Include '.flatpak' suffix in artifact name to avoid conflicts with other artifacts
• Fix cache to restore cache from correct arch

v6.4

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What's Changed

• builder: add upload-artifact flag to skip uploading artifacts by @​xTVaser in #​174
• Update deps by @​bilelmoussaoui in #​217

Full Changelog: flatpak/flatpak-github-actions@v6.3...v6.4

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