[9.18] fix: test: Use LC_ALL to override all system locales

The system tests were overriding the local locale by setting LANG to C.
This does not override the locale in case there are individual LC_<*>
variables like LC_CTYPE explicitly set.

Use LC_ALL=C instead which is the proper way of overriding all currently
set locales.

Backport of MR !9109

Merge branch 'backport-ondrej/use-LC_ALL-not-LANG-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9236