fpp.tmux: use $TMPDIR for temp directory
fpp.tmux: use $TMPDIR for temp directory

With fallback to /tmp if $TMPDIR is unset.
Closes #14

Signed-off-by: Joshua Boy Nicolai Appelman <d6955d9721560531274cb8f50ff595a9bd39d66f@jbna.nl>

tmux-plugins/tmux-fpp