build: Use meson’s post_install

Available since Meson 0.59.0:
https://mesonbuild.com/Gnome-module.html#gnomepost_install

Leaving package icon cache as is because the Meson tool
only supports the standard hicolor directory.
f424bfe4
1 job for wip/jtojnar/postinstall in 23 minutes and 55 seconds (queued for 1 second)