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

This reverts commit da9df7425a (libuv: win/spawn: run executables with
no file extension, 2023-11-29, v3.28.0-rc6~1^2~1).  It incorrectly
searched the `PATH` for extension-less command names.  Another fix will
be needed for the regression motivating it.

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.

Fixes: #25473
Issue: #25450