I've had a couple of people report issues like
https://github.com/wez/wezterm/issues/2378 recently, and we can avoid
this class of problem by unsetting SHELL at proces startup, so that's
what we do.
Since the original code was written, Rust grew and stabilized
an interface for specifying argv0 for the child process, so
we can remove the fragile `-l` argument passing.
refs: https://github.com/wez/wezterm/issues/753
