This makes the pushed artifact have the exact same hash if the contents
are the same.
E.g
```
flux push artifact oci://repo/image:tag1 --source deploy --revision="test" --path=deploy --reproducible
flux push artifact oci://repo/image:tag2 --source deploy --revision="test" --path=deploy --reproducible
```
will both result in the same sha hash, tagged with `tag1` and `tag2`.
This is useful when producing flux artifacts in a monorepo setup where
you don't want to unnecessarily push new artifacts unless something has
actually changed.
Signed-off-by: frekw <fredrik@warnsberg.se>
26109ee821