[devbox] Download nixpkgs using fetchTarball (#2639)

## Summary
Download nixpkgs using `fetchTarball`. We were previously using `fetchGit`, but it's too slow – users have been complaining that the first call to `devbox shell` takes to long, and part of it is due to `fetchGit`. Replacing with `fetchTarball` instead.

## How was it tested?
Re-built and ran on example projects.

## Is this change backwards-compatible?
Yes, I kept the same version of nixpkgs and everything.

tmpl/default.nix.tmpl

@@ -1,10 +1,9 @@
 let
-  pkgs = import (fetchGit {
-    url = "https://github.com/nixos/nixpkgs/";
-    ref = "refs/heads/nixos-unstable";
+  pkgs = import (fetchTarball {
     # Commit hash as of 2022-08-16
     # `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
-    rev = "af9e00071d0971eb292fd5abef334e66eda3cb69";
+    url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
+    sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
   }) {};
 in with pkgs;
 buildEnv {

tmpl/shell.nix.tmpl

@@ -1,10 +1,9 @@
 let
-  pkgs = import (fetchGit {
-    url = "https://github.com/nixos/nixpkgs/";
-    ref = "refs/heads/nixos-unstable";
+  pkgs = import (fetchTarball {
     # Commit hash as of 2022-08-16
     # `git ls-remote https://github.com/nixos/nixpkgs nixos-unstable`
-    rev = "af9e00071d0971eb292fd5abef334e66eda3cb69";
+    url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
+    sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
   }) {};
 in with pkgs;
 mkShell {