pin nixpkgs version in channel expression #31

Beginners often completely forget to set the nixpkgs channel to 20.03 when installing ARTIQ for the first time.
There are also sometimes unnecessary rebuilds when the nixpkgs version on client machines isn't exactly the same as the one it was built with on the server.

These issues can probably be fixed by pinning the exact nixpkgs version when generating the channel expression.

The artiq nixpkgs should be accessible from outside (e.g. as artiq-full.pkgs) so that other packages can be installed from the same version. It should also be possible to easily override the nixpkgs version used by artiq-full when importing it.

The ARTIQ manual should be updated accordingly.

Or maybe this:
https://www.tweag.io/blog/2020-05-25-flakes/
...which would potentially also avoid the generatedNix hack currently in use here.