ci: Stop passing -Dxdgmime-path to shared-mime-info

Since December, shared-mime-info has built xdgmime as a subproject, and no longer accepts the -Dxdgmime-path option.

So, add a symlink to its checked-out subprojects directory so that it uses the xdgmime we’re testing, and drop use of -Dxdgmime-path.

Signed-off-by: Philip Withnall pwithnall@gnome.org