setuid-wrappers: Update wrapper dir atomically.

[shlevy]

Fixes #18124.

[mention-bot]

@shlevy, thanks for your PR! By analyzing the annotation information on this pull request, we identified @edolstra, @MarcWeber and @peti to be potential reviewers

[shlevy]

@domenkozar please test

[edolstra]

Sorry, didn't see "please test" before hitting merge...

[domenkozar]

I'm still getting:

dummy> mv: cannot move '/var/setuid-wrappers' to '/var/setuid-wrappers-old': Device or resource busy.

[shlevy]

Hmm, even with /var/setuid-wrappers-old not existing already?

[shlevy]

(this may mean that this change has to be activated with nixos-rebuild boot)

[domenkozar]

Without:

[root@dummy:~]# ls /var/setuid*
setuid-wrappers.2DF8dF0aTv  setuid-wrappers.Pt01NyPpjG  setuid-wrappers.vaeo8U1fcw

[shlevy]

What the hell, where did those temp names come from?

[domenkozar]

I activated new configuration three times, this is what was left behind from the error probably.