Rename use-hermes-nightly workflow input to use-hermes-prebuilt (#56756)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56756

The use-hermes-nightly boolean input of prebuild-ios-core.yml was misleadingly named.
It controls whether to use the latest prebuilt Hermes from npm's latest-v1 dist-tag
(not a 'nightly' build). Rename:
- prebuild-ios-core.yml input: use-hermes-nightly -> use-hermes-prebuilt
- Update description of the input
- Update comment to move the TODO note inline
- Update callers in nightly.yml and test-all.yml

## Changelog:
[Internal] -

Reviewed By: cortinico

Differential Revision: D104649628

fbshipit-source-id: 71c75b07caebfe5e7d03e80d47cefba883843c9a