Apply nixfmt

[roberth]

Motivation

RFC 166

Now is a good time, because we're pretty much up to speed with backports.

I'll apply it to current maintenance branches as well, so that automated backports remain possible.

I will not add the automation to the maintenance branches though.

Context

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[edolstra]

I do wonder if we should format the lang tests. After all, the parser should also work (and give correct position info) on inputs that aren't formatted correctly. Probably doesn't matter though...

[roberth]

I do wonder if we should format the lang tests.

👍 I've excluded the lang/parse-* tests for this reason.
The other ones generally don't have syntax testing as a goal, but I'll double check.

Leaving them unformatted is annoying for contributors when enable format-on-save in their editor, so I think it's worth doing.

[roberth]

I've reviewed the formatted lang files as well as some others. Concretely, these are now also excluded:

tests/functional/lang/eval-fail-eol-3.nix
tests/functional/lang/eval-fail-set.nix
tests/functional/lang/eval-okay-comments.nix
tests/functional/lang/eval-okay-backslash-newline-2.nix
tests/functional/lang/eval-okay-ind-string.nix
tests/functional/shell.shebang.nix