os: double close pidfd if caller uses pidfd updated by os.StartProcess [1.23 backport]

[gopherbot]

@prattmic requested issue #68984 to be considered for backport to the next 1.23 minor release.

@gopherbot Please backport to 1.23. This is a regression in os.StartProcess for programs already using pidfds.

[gopherbot]

Change https://go.dev/cl/611415 mentions this issue: [release-branch.go1.23] os: dup pidfd if caller sets PidFD manually

[gopherbot]

Closed by merging CL 611415 (commit e6598e7) to release-branch.go1.23.