artiq-fast: pin nixpkgs, use in artiq-board-generated+artiq-full

attempt at gitea issue #31

artiq-board-generated/default.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {}
+{ pkgs ? import <artiq-fast/pkgs/nixpkgs.nix> { inherit (import <nixpkgs> {}) fetchgit; } {}
 , artiq-fast ? <artiq-fast>
 }:
 
@@ -22,7 +22,7 @@ let
     echo -n $HASH > $out/sinara-hash.txt
 
     cat > $out/default.nix << EOF
-    { pkgs ? import <nixpkgs> {}
+    { pkgs ? import ./fast/pkgs/nixpkgs.git { inherit (import <nixpkgs> {}) fetchgit; }
     }:
 
     let

artiq-fast/default.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {}}:
+{ pkgs ? import ./pkgs/nixpkgs.nix { inherit (import <nixpkgs> {}) fetchgit; } {}}:
 with pkgs;
 let
   artiq6 = pkgs.lib.strings.versionAtLeast mainPackages.artiq.version "6.0";

artiq-fast/pkgs/nixpkgs.nix

@@ -0,0 +1,7 @@
+{ fetchgit }:
+import (fetchgit {
+  url = "https://github.com/nixos/nixpkgs.git";
+  # nixos-21.09 as of 2021-03-08
+  rev = "1c7f02b0b9f115ccbc122ce39ae99debc1af857a";
+  sha256 = "14sichs3hyg4ybcacvzglw97lg747y1ls5lsazv0n5yxv5w06l3d";
+})

artiq-full.nix

@@ -1,5 +1,10 @@
-{ pkgs ? import <nixpkgs> {}
+{ a6p ? <a6p>
-, a6p ? <a6p>
+, artiq-fast ? (
+    if a6p
+    then <artiq-board-generated/fast>
+    else <artiq-fast>
+  )
+, pkgs ? import (artiq-fast + "/pkgs/nixpkgs.nix") { inherit (import <nixpkgs> {}) fetchgit; } {}
 }:
 
 let
@@ -13,10 +18,6 @@ let
       sha256 = sinaraSystemsHash;
     }
     else <sinaraSystemsSrc>;
-  artiq-fast =
-    if a6p
-    then <artiq-board-generated/fast>
-    else <artiq-fast>;
   artiqVersion = import (artiq-fast + "/pkgs/artiq-version.nix") {
     inherit (pkgs) stdenv git fetchgit;
   };
@@ -74,7 +75,7 @@ let
         HASH=`nix-hash --type sha256 --base32 $SINARA_SRC_CLEAN`
       ''}
     cat > $out/default.nix << EOF
-    { pkgs ? import <nixpkgs> {}}:
+    { pkgs ? import ./fast/pkgs/nixpkgs.nix { inherit (import <nixpkgs> {}) fetchgit; } {} }:
 
     let
       artiq-fast = import ${if a6p then "./board-generated" else "."}/fast { inherit pkgs; };