Document nix 2.18 requirement; disable nix setup test for nix 2.6

Since August 2025, nixpkgs requires a nix version of at least nix 2.18.
Consequently, our nix setup tests using nix 2.6.1 and Ubuntu 22 (nix 2.6.0)
break.

This comment documents that at least nix 2.18 is required, updates
the nix test to 2.18.0, and (temporarily) disables the Ubuntu 22 test.

Signed-off-by: Matthias J. Kannwischer <[email protected]>

Author: mkannwischer

.github/workflows/nix.yml

@@ -88,12 +88,15 @@ jobs:
       fail-fast: false
       matrix:
         target:
+          # nixpkgs requires 2.18 since August 2025, see 
+          # https://github.com/NixOS/nixpkgs/pull/428076
+          # TODO: Re-enable tests on Ubuntu 22 once nix has been updated to >= 2.18
+          # - runner: ubuntu-22.04
+          #   container:
+          #   install: 'apt'
           - runner: ubuntu-latest
-            container: nixos/nix:2.6.1
+            container: nixos/nix:2.18.0
             install: 'native'
-          - runner: ubuntu-22.04
-            container:
-            install: 'apt'
           - runner: ubuntu-24.04
             container:
             install: 'apt'

CONTRIBUTING.md

@@ -9,7 +9,7 @@ any of the open issues. Here are some things to get you started.
 
 We specify the development environment for mldsa-native using `nix`. If you want to help develop mldsa-native, please
 use `nix`. We recommend using the latest Nix version provided by the [nix installer
-script](https://nixos.org/download/), but we currently support all Nix versions >= 2.6.
+script](https://nixos.org/download/), but we currently support all Nix versions >= 2.18.
 
 All the development and build dependencies are specified in [flake.nix](flake.nix). To execute a bash shell, run
 ```bash