libuv: Revert "win/spawn: run executables with no file extension"

This reverts commit fc6231bee5 (libuv: win/spawn: run executables with
no file extension, 2023-12-04, v3.28.0~9^2) to make room for the
corresponding fix from upstream libuv 1.48.

Record this as a merge from the last-imported upstream libuv snapshot
branch so that future `git blame` points to the upstream for the
original code instead of this commit.

Issue: #25450