add `--without-download` to `update-stemcell` command if possible

[dlinsley]

Is your feature request related to a problem? Please describe.

With kiln update-release we have an option: --without-download which will just check the existence and checksum of releases in the remote release source

kiln update-stemcell does not have this option currently and requires downloading all of the respective releases for the new stemcell to determine the checksum of each compiled release against the new stemcell.

Describe the solution you'd like

By having kiln update-stemcell --without-download we can check the release source for existence and checksum of the compiled release without downloading them when compiled releases for a new stemcell are available.

Describe alternatives you've considered

No response

Additional context

No response