feat(build): Add dependencies for custom repo clones

Adds git & SSH as part of the Nixery image, which are required to use
Nix's builtins.fetchGit.

The dependency on interactive tools is dropped, as it was only
required during development when debugging the image building process
itself.

default.nix

@@ -94,13 +94,13 @@ rec {
     name = "nixery";
     config.Cmd = ["${nixery-launch-script}/bin/nixery"];
     contents = [
-      bashInteractive
       cacert
-      coreutils
+      git
-      nix
-      nixery-launch-script
       gnutar
       gzip
+      nix
+      nixery-launch-script
+      openssh
     ];
   };
 }