Fix nix-prefetch-git to work from within the nix build

[balsoft]

This PR adds a patched nix-prefetch-git so that uncached dependencies can be downloaded using IFD (with --no-sandbox, of course). Fixes #335

[angerman]

I’d rather see this as a patch over nix-prefetch-git than an outright new script. That way staying current with upstream as well as making the changes explicit are given.

[balsoft]

Actually, this probably should be fixed in a different way. See https://gitlab.com/tezosagora/agora/commit/330ae03ed732b8843c94be3724dd4637178091b1 , where @yorickvP fixed this problem so that --no-sandbox is not required. I can totally see how we can move this code to haskell.nix (so that cache is generated automatically)

[angerman]

@balsoft so you are saying we should abandon this PR, in favour of one that generates?

[balsoft]

@angerman it's up to you to decide which is better. I can fix this so that it's a patch instead of whole script replacement, or I can move what @yorickvP has done to here or to nix-tools so that we get sandboxed builds.

[angerman]

Closed in favour of #358