sort: ignore --parallel on WASI

[cakebaker]

While reviewing #13024 I noticed that we don't use the value of --parallel on WASI because WASI doesn't support threads.

So this PR ignores the value of --parallel on WASI. In addition, it removes the threads property from GlobalSettings and uses a local variable instead.

[oech3]

There is wasm32-wasip1-threads which is not used by us yet.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!