30_env.ax-io: Try to be even more clever when setting TMPDIR
30_env.ax-io: Try to be even more clever when setting TMPDIR

Silently use the "runtime directory" of the user, when this is set already.

And if not, make sure to not use "/" but fallback to "/tmp" when the mktemp(1)
command failed for example.

alexbarton/ax-zsh