In order to have an UTF-8 locale in the build root. This is something
normally set on real machines but is not set from our chroots. Meson,
for example, loudly complains when the locale charset is not UTF-8.

I'd like to have C.UTF-8, as most other distributions do. Unfortunately,
it's not part of vanilla glibc; en_US.UTF-8 will have to do.

mkarchroot already creates roots with both en_US.UTF-8 and de_DE.UTF-8,
the latter because builds of gcc (perhaps used to) require it.

Bump the CHROOT_VERSION due to the setting change.