fetchgit: call in-repository script with bash explicitly

The script's shebang depends on /usr/bin/env, which we don't have in chroot
environments. This patch remedies the fallout from ade9f7167d, which
fixed https://github.com/NixOS/nixpkgs/issues/11284.

pkgs/build-support/fetchgit/default.nix

@@ -45,7 +45,7 @@ assert deepClone -> leaveDotGit;
 stdenv.mkDerivation {
   inherit name;
   builder = ./builder.sh;
-  fetcher = ./nix-prefetch-git;
+  fetcher = "${stdenv.shell} ${./nix-prefetch-git}";
   buildInputs = [git];
 
   outputHashAlgo = if sha256 == "" then "md5" else "sha256";