Pin nixpkgs version

[jethrogb]

Trying to do the reproducible build right now results in the following error in step 10 of the Dockerfile:

error: evaluation aborted with the following error message: '
       This version of Nixpkgs requires an implementation of Nix with the following features:
       - `builtins.nixVersion` reports at least 2.18

       Your are evaluating with Nix 2.9.0, please upgrade:

       - If you are running NixOS, `nixos-rebuild' can be used to upgrade your system.

       - Alternatively, with Nix > 2.0 `nix upgrade-nix' can be used to imperatively
         upgrade Nix. You may use `nix-env --version' to check which version you have.

       - If you installed Nix using the install script (https://nixos.org/nix/install),
         it is safe to upgrade by running it again:

             curl -L https://nixos.org/nix/install | sh

       For more information, please see the NixOS release notes at
       https://nixos.org/nixos/manual or locally at
       /nix/store/6hg0kq5x9sdma1bmva3qj18zbqdpp5q5-nixpkgs/nixpkgs/nixos/doc/manual/release-notes.

       If you need further help, see https://nixos.org/nixos/support.html
       '
(use '--show-trace' to show detailed location information)
The command '/bin/sh -c . /home/user/.nix-profile/etc/profile.d/nix.sh && nix-shell' returned a non-zero code: 1
Unable to find image 'sgx.build.env:latest' locally
docker: Error response from daemon: pull access denied for sgx.build.env, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.

The problem seems to be that nixpkgs is not pinned and always using the latest version. This PR pins the nixpkgs version to 22.05, which is the version that was available when Nix 2.9 was released.

More details about nixpkgs pinning:
https://nix.dev/tutorials/first-steps/towards-reproducibility-pinning-nixpkgs.html
https://nix.dev/reference/pinning-nixpkgs.html

[jethrogb]

Strangely enough, with this change I still get a similar error:

error: evaluation aborted with the following error message: '
       This version of Nixpkgs requires an implementation of Nix with the following features:
       - `builtins.nixVersion` reports at least 2.18

       Your are evaluating with Nix 2.9.0, please upgrade:

       - If you are running NixOS, `nixos-rebuild' can be used to upgrade your system.

       - Alternatively, with Nix > 2.0 `nix upgrade-nix' can be used to imperatively
         upgrade Nix. You may use `nix-env --version' to check which version you have.

       - If you installed Nix using the install script (https://nixos.org/nix/install),
         it is safe to upgrade by running it again:

             curl -L https://nixos.org/nix/install | sh

       For more information, please see the NixOS release notes at
       https://nixos.org/nixos/manual or locally at
       /nix/store/6hg0kq5x9sdma1bmva3qj18zbqdpp5q5-nixpkgs/nixpkgs/nixos/doc/manual/release-notes.

       If you need further help, see https://nixos.org/nixos/support.html
       '
will use bash from your environment

However, the build continues and I'm able to reproduce the most recent AEs.