whatever default shell is used despite it is not a posix compatible shell

I set default shell to fish and saw btrbk using fish. fish is not a POSIX compatible shell so it should not be used by btrbk.

Start by reproducing the issue with fish configured as the default shell and observe how btrbk invokes shell commands. Trace the relevant command entry point to determine why the default shell is selected. Done means btrbk consistently uses a POSIX-compatible shell instead of fish, with regression coverage for this case.

Written by the indexing model from the issue text.