Save mutation output as an artifact

[George Karachalias]

The mutate action runs mutation tests and the results can be seen in the output log of the GitHub Action. However, since we've switched to using earthly and earthly limits the output log (to ~2MB at the moment I believe), logs can be pruned; the untested mutations appear at the end of the log so they get lost. This can be avoided if we save the output of the mutation tests as an artifact, instead of relying on the logs.

[George Karachalias]

The mutate action has been removed (9b59ce4 and dd0833a) so this is no longer relevant. We can always run mutations locally when desirable. Closing.