rc: Export $TMPDIR to new tmux processes

`tmux` will start new processes (e.g. when creating panes or windows)
with the same environment it was started with, which means that if the
$TMPDIR variable was overriden for the kakoune server from within
`tmux`, newly created panes/windows won't have access to the server
socket to sustain a session.

This commit fixes the issue by always exporting the $TMPDIR variable
from the parent `tmux` environment to the new processes.

Fixes #1319

rc/base/tmux.kak

@@ -24,7 +24,7 @@ def -hidden -params 1.. tmux-new-impl %{
         tmux_args="$1"
         shift
         if [ $# -ne 0 ]; then kakoune_params="-e '$@'"; fi
-        TMUX=$tmux tmux $tmux_args "kak -c ${kak_session} ${kakoune_params}" < /dev/null > /dev/null 2>&1 &
+        TMUX=$tmux tmux $tmux_args "TMPDIR='${TMPDIR}' kak -c ${kak_session} ${kakoune_params}" < /dev/null > /dev/null 2>&1 &
     }
 }