format: ignore system and user git config

This can cause failures if the user has configuration that is
incompatible with the version of git pinned in the format script.

Potentially it could have other undesired effects as well.

Fix #5736

format

@@ -1,6 +1,11 @@
 #! /usr/bin/env nix-shell
 #! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/6616de389ed55fba6eeba60377fc04732d5a207c.tar.gz -i bash -p git gnugrep gnused findutils nixfmt
 
+# Avoid being affected by system and user git config.
+export GIT_CONFIG_NOSYSTEM=1
+export HOME=
+export XDG_CONFIG_HOME=
+
 nixfmt_args=()
 files=()