fuzzing: use LC_ALL instead of LC_COLLATE

[cakebaker]

The output of GNU sort in #5849 doesn't seem to use the C locale, even though LC_COLLATE=C is set before running GNU sort. The help says to set LC_ALL=C:

The locale specified by the environment affects sort order.
Set LC_ALL=C to get the traditional sort order that uses
native byte values.

On my machine I don't see a difference, both LC_COLLATE=C and LC_ALL=C lead to the same sort output. So I'm not sure whether this PR will fix the issue.