darwin-installer: add missing shebang

Unclear in which conditions this does or doesn't work but using a
shebang is best practice and ensures an ancient version of bash isn't
used.

Fixes #179

pkgs/darwin-installer/default.nix

@@ -24,6 +24,7 @@ stdenv.mkDerivation {
   '';
 
   shellHook = ''
+    #!${stdenv.shell}
     set -e
 
     _PATH=$PATH

pkgs/darwin-uninstaller/default.nix

@@ -22,7 +22,7 @@ stdenv.mkDerivation {
   '';
 
   shellHook = ''
-    #!/usr/bin/env bash
+    #!${stdenv.shell}
     set -e
 
     action=switch