cp: fix crash on -T/--no-preserve-target

[Alonely0]

This pull request fixes an edge case in which cp would crash when provided the -T flag, and also fixes a logic bug that arose in this edge case. Furthermore, a test case has been added to improve the test coverage of this flag. This PR fixes #8708.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)

[Alonely0]

Interesting how CI is having an embolism over supported features on our MSRV (still has an outdated compiler cached?) but also about some random package (outdated ubuntu repos?)? The former is not a big deal because I can just reverse the latest commit, which is the one actually needs 1.85.0; the later, however...

EDIT: I was gaslighted by the internet yesterday, ima revert the commit.

[cakebaker]

@Alonely0 the issue with the outdated ubuntu repo should be fixed with #8865

[Alonely0]

@Alonely0 the issue with the outdated ubuntu repo should be fixed with #8865

Tysm. Let's see if it complains about let-chains (been finding conflicting info on stabilization status) and if it does I'll push my local branch with the commit reverted.

[cakebaker]

@Alonely0 let chains have been introduced with Rust 1.88, see https://blog.rust-lang.org/2025/06/26/Rust-1.88.0/#let-chains

[Alonely0]

Yeah, done. I've removed the unwrap call anyway; I don't feel comfortable with those.

[cakebaker]

Thanks for your PR!