Pin and cache nixpkgs commit archive

[21e8]

Hi!

I noticed that this step often takes a while, especially on smaller instances:

#8 2.063 unpacking 'https://github.com/NixOS/nixpkgs/archive/5624e1334b26ddc18da37e132b6fa8e93b481468.tar.gz' into the Git cache...

my suggestion would be to be able to cache this archive and let the user pin it the commit.
that way we can speed up deployments, especially for small apps on small instances, by a lot.

happy to make suggestions but will be out for a few weeks from tomorrow on.

kind regards!