ci: use same pinned nixpkgs for hardware tests

ci/hardware_tests/shell.nix

@@ -1,4 +1,4 @@
-with import <nixpkgs> {};
+with import ../nixpkgs.nix;
 
 stdenv.mkDerivation rec {
   name = "trezor-firmware-hardware-tests";

ci/nixpkgs.nix

@@ -0,0 +1,5 @@
+# the last successful build of nixpkgs-unstable as of 2020-12-30
+import (builtins.fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/bea44d5ebe332260aa34a1bd48250b6364527356.tar.gz";
+  sha256 = "14sfk04iyvyh3jl1s2wayw1y077dwpk2d712nhjk1wwfjkdq03r3";
+}) { }

ci/shell.nix

@@ -1,13 +1,6 @@
 { fullDeps ? false }:
 
-# the last successful build of nixpkgs-unstable as of 2020-12-30
-with import
-  (builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/bea44d5ebe332260aa34a1bd48250b6364527356.tar.gz";
-    sha256 = "14sfk04iyvyh3jl1s2wayw1y077dwpk2d712nhjk1wwfjkdq03r3";
-  })
-{ };
-
+with import ./nixpkgs.nix;
 let
   moneroTests = fetchurl {
     url = "https://github.com/ph4r05/monero/releases/download/v0.17.1.9-tests/trezor_tests";