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 and there are no
pratcically relevant distribution versions around with older versions

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