Handle missing /usr/share/desktop-directories on installation

Error message:

xdg-desktop-menu: No writable system menu directory found.
dpkg: error processing package briar-desktop (--install):
 installed briar-desktop package post-installation script subprocess returned error exit status 3
Errors were encountered while processing:
 briar-desktop

workaround found here:

sudo mkdir /usr/share/desktop-directories/