fix: make image update checks use less registry quota

[pelluch]

This makes image update checks use

docker buildx imagetools inspect --raw

instead of

docker buildx imagetools inspect --format "{{json .Manifest}}"

Results are identical, but as explained in linked issue, it uses significantly less quota, measured with at least Docker Hub.

Note that the Cargo.lock diff here shows urlencoding and formatting were removed. Those were not a manually-changed part of this PR, but it happened automatically, presumably because some earlier change didn't properly re-generate / commit that change.

Tested via a local dev instance, which showed checks still working, and using less quota (as evidenced via the Docker Hub apis mentioned in the linked issue).

Resolves #1652