shells (artiq-fast): change artiqpkgs, vivado to arguments

This allows overriding the artiqpkgs used downstream.
The issue is that previously artiqpkgs required <artiqSrc> to be defined,
and overriding that required setting paths, added complexity, and possible impurities.

This solution allows reuse of the shell[-dev], from a given version of
ARTIQ, without necessarily specifying source or a local source repo.

artiq-fast/shell-dev.nix

@@ -1,9 +1,8 @@
-{ pkgs ? import <nixpkgs> {}}:
+{ pkgs ? import <nixpkgs> {}
+, artiqpkgs ? import ./default.nix { inherit pkgs; }
+, vivado ? import ./vivado.nix { inherit pkgs; }
+}:
 
-let
-  artiqpkgs = import ./default.nix { inherit pkgs; };
-  vivado = import ./vivado.nix { inherit pkgs; };
-in
 assert pkgs.lib.asserts.assertMsg (!pkgs.lib.strings.versionAtLeast artiqpkgs.artiq.version "7.0") "For ARTIQ 7+, use 'nix develop' on the flake instead.";
 pkgs.mkShell {
   buildInputs = [

artiq-fast/shell.nix

@@ -1,8 +1,7 @@
-{ pkgs ? import <nixpkgs> {}}:
+{ pkgs ? import <nixpkgs> {}
+, artiqpkgs ? import ./default.nix { inherit pkgs; }
+}:
 
-let 
-  artiqpkgs = import ./default.nix { inherit pkgs; };
-in
 pkgs.mkShell {
   buildInputs = [ (pkgs.python3.withPackages(ps: [artiqpkgs.artiq])) ];
 }