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

src/libexpr-test-support/package.nix

@@ -1,15 +1,16 @@
-{ lib
-, mkMesonLibrary
+{
+  lib,
+  mkMesonLibrary,
 
-, nix-store-test-support
-, nix-expr
-, nix-expr-c
+  nix-store-test-support,
+  nix-expr,
+  nix-expr-c,
 
-, rapidcheck
+  rapidcheck,
 
-# Configuration Options
+  # Configuration Options
 
-, version
+  version,
 }:
 
 let