Format .nix files

This does not include any automation for the release branch, but
is based on the configuration of https://github.com/NixOS/nix/pull/12349

    pre-commit run -a nixfmt-rfc-style

tests/functional/lang/eval-okay-string.nix

@@ -1,12 +1,13 @@
-"foo" + "bar"
-  + toString (/a/b + /c/d)
-  + toString (/foo/bar + "/../xyzzy/." + "/foo.txt")
-  + ("/../foo" + toString /x/y)
-  + "escape: \"quote\" \n \\"
-  + "end
+"foo"
++ "bar"
++ toString (/a/b + /c/d)
++ toString (/foo/bar + "/../xyzzy/." + "/foo.txt")
++ ("/../foo" + toString /x/y)
++ "escape: \"quote\" \n \\"
++ "end
 of
 line"
-  + "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"
-  + "foo$bar"
-  + "$\"$\""
-  + "$"
++ "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"
++ "foo$bar"
++ "$\"$\""
++ "$"