This allows running code to change the environment before the
nix-prefetch-git script is run. In particular, it allows setting things
like NIX_PREFETCH_GIT_CHECKOUT_HOOK, which allows additional code to be
run before deleting the .git directory.
This also means there's potentially value in performing a deep clone
then removing the .git directory, so remove the assertion that prevents
doing so.
d95d3724d8