fetchgit: follow up to 2cf7069b7d

If "fetcher" is a string, then Nix will execute it with bash already, so
the additional bash argument in that string was redundant and apparently
causes trouble on non-Linux platforms.

Hopefully fixes https://github.com/NixOS/nixpkgs/issues/11496.

pkgs/build-support/fetchgit/default.nix

@@ -45,7 +45,7 @@ assert deepClone -> leaveDotGit;
 stdenv.mkDerivation {
   inherit name;
   builder = ./builder.sh;
-  fetcher = "${stdenv.shell} ${./nix-prefetch-git}";
+  fetcher = "${./nix-prefetch-git}";  # This must be a string to ensure it's called with bash.
   buildInputs = [git];
 
   outputHashAlgo = if sha256 == "" then "md5" else "sha256";