I've run into issues with bash completion failing when using `nix` to
handle environments.
The nix environment often installs its own `bash` which doesn't respect
the `BASH_COMPLETIONS` env-var.
This allows an (off-by-default) option to override what is returned by
`xonsh.platform.bash_command`.
The removal of the `lru_cache` should be negligible since
`windows_bash_command` has its own `lru_cache` and the other options are
doing an env lookup or returning a string.
