Fix coupled restarts in global coupled model

### Are there any linked Issues or Pull Requests?

MetOffice/lfric_core#273

### Brief description

As outlined in the parent issue #233, https://code.metoffice.gov.uk/trac/lfric_apps/ticket/981 broke restart capability with the coupled model. This became visible when trying to port GC6 to LFRic Apps vn3.0, #196.

This issue captures the LFRic Apps fix in the parent issue for the global coupled model restart as submitted by GMED, see the implementation in the FCM branch at the git_migration: https://code.metoffice.gov.uk/trac/lfric_apps/browser/main/branches/dev/timgraham/git_migration_fix_cpl_checkpoint.

### Further details of the issue.

_No response_