add git to installer environment

there seems to be a hidden dependency on `git` in `builtins.fetchGit`:
<NixOS/nixpkgs#46603>

<NixOS/nixpkgs#46603 (comment)>:

> > This is intentional. git is not a hard dependency since it's only
    needed if you use fetchGit. Since Git is a sizeable dependency, we
    don't want to pull it in unconditionally. (Likewise for Mercurial).

> This line of argument seems to throw the entire purpose and
  practicality of nix into question...

I completely agree.

install

@@ -1,5 +1,5 @@
 #! /usr/bin/env nix-shell
-#! nix-shell -i fish -p fish
+#! nix-shell -i fish -p fish -p git
 
 set current (dirname (readlink -f (status -f)))
 # TODO: proper command line options, e.g. `install <machine> [--root <root>]`