refactor(mu4e): use doom-call-process to check XDG
refactor(mu4e): use doom-call-process to check XDG

This is a bit safer than shell-command-to-string, and we also now check
that the command exited successfully, and fall back if not.

hlissner/.emacs.d,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs