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/repl/doc-comment-function.nix

@@ -1,3 +1,4 @@
-/** A doc comment for a file that only contains a function */
-{ ... }:
-{ }
+/**
+  A doc comment for a file that only contains a function
+*/
+{ ... }: { }