This improves the hard-coded list of ARM fallbacks in the `platform.Only` implementation (by doing a descending loop over variant numbers instead, which is all the hard-coded list was doing).
Making this a separate function can then more easily be recursive later for handling an `arm64`->`arm` fallback (or similar), but I think it makes the code a lot more clear too (so we're calculating a vector of platforms separately from building a matcher object).
This also makes a minor adjustment in `TestImagePullWithDistSourceLabel` which had an implicit assumption that `platforms.Only` would only ever result in a single suitable manifest, which isn't strictly true (and is likely failing as-is when run on any 32bit `arm` system that's `v6` or higher, which this fixes 😅).
Signed-off-by: Tianon Gravi <admwiggin@gmail.com>
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