os/exec: If GOOS=windows, executing executable files in the current directory fails [1.21 backport]

[gopherbot]

@bcmills requested issue #61951 to be considered for backport to the next 1.21 minor release.

@gopherbot, please backport to Go 1.21. This is a regression from previous Go releases.

[bcmills]

This appears not to be a regression after all.