FindSharedMimeInfo: Also run update-mime-database with DESTDIR

I am currently trying to cross-compile various programs for CheriBSD (a fork of FreeBSD that supports CHERI-enabled architectures). As part of my build scripts I use DESTDIR for installation which means the shared mime info cache is not being generated. As I run everything on QEMU, I would like to avoid having to parse large XML files and instead use the cache. I can obviously add a manual step after building the project but having CMake do it automatically is nicer.