Preserve Nix CLI and flakes features in the CI installer

.github/workflows/nix.yml

@@ -80,8 +80,9 @@ 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' || '' }}
+            # The installer skips its defaults when this setting is present.
+            # Keep the CLI/flakes features and enable the marker's Nix feature.
+            experimental-features = nix-command flakes ${{ matrix.system == 'x86_64-linux' && 'recursive-nix' || '' }}
             extra-system-features = ${{ matrix.system == 'x86_64-linux' && 'recursive-nix' || '' }}
 
       - name: Show outputs from this checkout

scripts/nix/README.md

@@ -171,7 +171,10 @@ builder for those jobs; keep it off Rosetta slots. Nix also requires the
 when building derivations with this reserved marker. Enable both
 `extra-experimental-features = recursive-nix` and
 `extra-system-features = recursive-nix` on that builder. The GitHub workflow
-sets both only for its native x86_64 Linux runner. The upstream fixes and regression jobs
+sets the complete `experimental-features` list, including `nix-command` and
+`flakes`, because its installer suppresses those defaults when this setting is
+present. It adds `recursive-nix` to both lists only for its native x86_64 Linux
+runner. The upstream fixes and regression jobs
 are in [haskell.nix PR #2585](https://github.com/input-output-hk/haskell.nix/pull/2585).
 
 To build both Android APK ABIs from the current checkout, preserving the same