ci: install nix for sign tarball action

.github/workflows/release-tip.yml

@@ -97,6 +97,13 @@ jobs:
     runs-on: namespace-profile-ghostty-md
     steps:
       - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v30
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@v15
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
       - name: Create Tarball
         run: git archive --format=tgz -o ghostty-source.tar.gz HEAD
       - name: Sign Tarball