installer: Handle locales with '.UTF-8' suffix
installer: Handle locales with '.UTF-8' suffix

Apparently its not just '.utf8' but also '.UTF-8' suffix thats used as
suffix in locale names. Better make sure we strip that too before giving
the string to libosinfo.

https://bugzilla.gnome.org/show_bug.cgi?id=700415

vbenes/gnome-boxes,vbenes/gnome-boxes