cask/caskroom: harden Linux Caskroom group lookup

- Memoize `expected_caskroom_group` to avoid repeated `Etc.getgrgid` calls.
- Rescue `ArgumentError` to cover misconfigured NSS setups, matching the
  pattern in `Utils::UID.uid_home`.
- Stub `Etc.getgrgid` in the Linux spec for determinism and reset the
  memoization between examples.