Handle PS1 on Tastycake

On Tastycake, __git_ps1 doesn't work as part of PROMPT_COMMAND as
expected.  I can't immediately see what's going on, so for now, use a
regular PS1 and cope with the fact that the Git part isn't going to be
coloured in.

(cherry picked from commit 1d516367cfc10a2ca25fae7079e0a50a306b36d6)