Enable the Nix feature required by guarded cross slices

.github/workflows/nix.yml

@@ -80,6 +80,8 @@ jobs:
             extra-trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= loony-tools:pr9m4BkM/5/eSTZlkQyRt57Jz7OMBxNSUiMC4FkcNfk=
             # The pinned haskell.nix uses this scheduling marker for cross
             # slices that need a real x86_64 host. This runner is native x86_64.
+            # Nix also requires its experimental feature for this marker.
+            extra-experimental-features = ${{ matrix.system == 'x86_64-linux' && 'recursive-nix' || '' }}
             extra-system-features = ${{ matrix.system == 'x86_64-linux' && 'recursive-nix' || '' }}
 
       - name: Show outputs from this checkout