fix(maestro): make the flake CI job work on the runner

Two defects in the job as landed, both invisible locally:

- The multi-user installer needs systemd, which the act_runner container
  does not have. Single-user install instead.
- The installer does not enable flakes, so `nix build .#…` fails on a
  fresh install. A machine with them in nix.conf builds fine, which is
  why this passed locally and failed in CI.

Refs #98

.gitea/workflows/ci.yml

@@ -106,14 +106,19 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      # Single-user install: the runner is a container with no systemd, and
+      # the --daemon installer refuses without it.
       - name: Install nix
         run: |
-          curl -fsSL https://nixos.org/nix/install | sh -s -- --daemon --yes
-          echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
+          curl -fsSL https://nixos.org/nix/install | sh -s -- --no-daemon --yes
+          echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
 
+      # The installer does not enable flakes, so `nix build .#…` needs the
+      # features passed explicitly — a local machine that has them in
+      # nix.conf will build fine while CI fails without this.
       - name: build flake packages
         run: |
-          nix build \
+          nix --extra-experimental-features "nix-command flakes" build \
             .#webFrontend \
             .#maestro \
             .#maestro-web \