execute_process: Revert accidental search of PATH for no extension on Windows

Revert !9017 (merged) because it incorrectly searched the PATH for extension-less command names. This was exposed by failures in nightly testing that we've had since !9017 (merged) was staged/merged, but which were mistaken for spurious failures at the time.

Replace the change with a new backport from a revised libuv PR 4241 to retain the intended fix without the new regression.

Fixes: #25473 (closed)
Issue: #25450 (closed)
Backport: release