qa: unset LC_ALL in the check script to fix locale-dependent sort order

[kurik]

When LC_ALL is set in the caller's environment (e.g. LC_ALL=en_US.utf8),
it overrides any per-command LC_COLLATE=POSIX setting used inside
individual QA tests, causing locale-sensitive sort ordering instead of
the expected POSIX byte-value ordering. This manifests as spurious
output mismatches in tests such as 821/1663 where metric names are
enumerated via "LC_COLLATE=POSIX sort".
Unsetting LC_ALL at the start of the check script removes the inherited
override while still allowing individual tests to apply their own
per-command locale settings (e.g. "LC_COLLATE=en_US.utf8 sort") as
intended by their authors.

[kurik]

@kmcdonell May I ask you please for a review, to confirm this idea?
It hit me when I saw many tests to fail on my downstream test-bed , just because by an accident the LC_ALL was propagated from my workstation to the test-bed global environment.