use writeFileAtomic in Distribution.Client.Config

[ulysses4ever]

see haskell#11759

in particular this comment:

It adds some more cleanup than strictly what haskell#11686 asks for (the teardown function removes some extra *.tmp files) but it looks legit: it addresses this logic in Distribution.Client.Config

Rather than work around it in the test suite, let's fix Distribution.Client.Config: the shenanigans with tmpFile are ill advised, just createDirectoryIfMissing and immediately writeFileAtomic file.

haskell#11759 (comment)