ci: enable cgroups feature alongside auto-allocate-uids

Nix needs both experimental features (auto-allocate-uids + cgroups)
plus the use-cgroups setting for the nspawn container tests to run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kmein

.github/workflows/ci.yml

@@ -12,10 +12,11 @@ jobs:
 
       - uses: DeterminateSystems/nix-installer-action@main
         with:
-          # nspawn container tests need auto-allocate-uids + uid-range.
+          # nspawn container tests need auto-allocate-uids + cgroups.
           extra-conf: |
-            extra-experimental-features = auto-allocate-uids
+            extra-experimental-features = auto-allocate-uids cgroups
             auto-allocate-uids = true
+            use-cgroups = true
 
       - uses: DeterminateSystems/magic-nix-cache-action@main