install nix myself

.gitea/workflows/nix-flake-check.yaml

@@ -7,5 +7,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
-        uses: cachix/install-nix-action@v27
-      - run: nix flake check --all-systems
+        run: |
+          sh <(curl -L https://nixos.org/nix/install) --no-daemon --no-channel-add
+          mkdir -p ~/.config/nix
+          touch ~/.config/nix/nix.conf
+          echo "max-jobs = auto" >> ~/.config/nix/nix.conf
+          echo "cores = 0" >> ~/.config/nix/nix.conf
+          echo "experimental-features = nix-command flakes" >> ~/.config/nix/nix.conf
+          echo "/nix/var/nix/profiles/per-user/$USER/profile/bin" >> "$GITHUB_PATH"
+          echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
+      - name: Check flake
+        run: nix flake check --all-systems
+