x11-drivers/nvidia-drivers: Fix conftest.sh locale expectations

x11-drivers/nvidia-drivers: Fix conftest.sh locale expectations

conftest.sh in 440.* expects to find "$file: No such file or directory"
in compile test output for kernel headers, but with LC_ALL set, its
translations will not trigger that, resulting in false positives for
certain headers where the test intended to rule out false negatives.
Fix this by setting the default locale in conftest.sh.

Package-Manager: Portage-2.3.77, Repoman-2.3.17
Fixes: https://bugs.gentoo.org/698238
Signed-off-by: Jeroen Roovers <jer@gentoo.org>