invoke update-mime-database with -n

it only updates the database iff there's new files and is specifically meant for post-install invocation. since cmake only installs files when they actually changed this effectively turns repeated cmake --install calls noop as far as database updates are concerned

this stops all sources with mimetype definitions from rebuilding the mime database on every install even when nothing changed. it saves literal seconds every install call

-n was introduced in 1.4 which is like 6 years old, in the interest of backwards compatibility we'll only set it when appropriate though