Fix nix config option

[cbclemmer]

Closes #8452 I did not create a test for the config files when I made #8054. This should test config files for leaving out nix, "nix: True", and "nix: False". This also adds a line in the --help output for the nix option, it may not be necessary and can be easily removed if it is not needed.

@mhwombat it would be useful for you to confirm that this fix removes the error you were seeing @Mikolaj I assigned you to review, if this isn't correct, please redirect the review.

---

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• [x] Patches conform to the coding conventions.
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[cbclemmer]

But uh... let me get the tests passing firsts

[cbclemmer]

It looks like this pr for bytestring is causing the tests to fail. It looks like the test that failed depended upon getting the latest version of bytestring and then building it, it then tried to build the data-array-byte package but couldn't find it. I'm getting the same results locally when running the test command but I can build the bytestring repo locally just fine. Maybe this is an issue with the server caching packages or something? @Mikolaj What do you think?

[cbclemmer]

Relevant links: https://github.com/haskell/cabal/blob/master/cabal-testsuite/PackageTests/postCheckoutCommand/cabal.positive.project https://github.com/haskell/bytestring/pull/410 https://github.com/Bodigrim/data-array-byte

[cbclemmer]

@Mikolaj Tests are passing 🎉

[Mikolaj]

@cbclemmer: thank you for the diagnosis of the spurious CI failure. Indeed that seems to have been the problem and we have since fixed it after a lot of confusion. A pity I haven't looked at this PR earlier. :)

@mhwombat, might we solicit your review? Even just an absent-minded look at the code, a checkout, a run and a report in a comment that it works fine now? I will have a closer look now, but another pair of eyes is very welcome (and also we formally require two reviews for each PR).

[Mikolaj]

The CI failure is probably caused by https://bugs.launchpad.net/ubuntu/+source/git/+bug/1993586 (and by us using the latest Ubuntu container).

[Mikolaj]

May be fixed in https://github.com/haskell/cabal/pull/8546 (if CI passes there).

[Mikolaj]

The CI fix seems to work. Let me rebase on it...

[Mikolaj]

@mergify rebase

[mergify[bot]]

rebase

✅ Branch has been successfully rebased

[cbclemmer]

@Mikolaj Thank you, I was really confused

[Mikolaj]

This time it seems it failed due to some corruption, hanging and cancellation after 360min. I'm restarting.

[Mikolaj]

Succes. Quick, @cbclemmer, set one of the merge_me labels to merge the PR while it still passes CI. :)

[Mikolaj]

Edit: with mergify waiting 2 days after the label is set, as per the process.