$VIMRPLUGIN_TMPDIR is preferentially created in $TMPDIR

Avoids problems when /tmp is not available or non-writeable. $TMPDIR (if set) is vim's preferred directory for temporary files, see :help tempfile.
In my specific case, vim and R run on two different hosts (R is executed on a remote host via a wrapper script) that mount the same filesystem for /home but not for /tmp. This results in vimcom complaining that '/tmp/r-plugin-username/vimcom_running is not writeable.