[libc++] [test] Recommit the unsetting of LC_COLLATE in the builder script.

This re-reverts one piece of 1b885573,
reapplying one piece of D101437 (but not the "service"-queue piece of it).

It turns out that the behavior of `grep [^ -~]`, or even `grep [A-Z]`,
depends on locale, specifically `LC_COLLATE`. So we want to make sure
we're not in any weird locale, no matter what machine we're running on.
Yes, "en_US.UTF-8" counts as weird!
https://stackoverflow.com/questions/67320156/misbehavior-of-gnu-grep-when-grepping-for-ignores-spaces
https://stackoverflow.com/questions/6799872/how-to-make-grep-a-z-independent-of-locale