update keys script to use core release artefacts

[turbocrime]

after penumbra-zone/penumbra#4130 penumbra-zone/penumbra#4195 it is possible to download a tarball containing keys with checksum in the release manifest, rather than a curling a glob and checking a manually-generated manifest.

keys script should transition to using the official release artefact.

[turbocrime]

moved to todo since we discussed in meeting as todo but the issue didn't actually exist

[grod220]

Not a high priority, can revisit this after mainnet