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 commit 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'