Try to install attic for gitea builder

.gitea/workflows/check-flake.yaml

@@ -15,6 +15,8 @@ jobs:
         with:
           fetch-depth: 0
 
+      - run: attic -V
+
       - name: Setup Attic Cache
         uses: https://github.com/ryanccn/attic-action@v0
         with:

common/server/gitea-actions-runner.nix

@@ -116,6 +116,7 @@ in
       git
       # Gitea Actions rely heavily on node. Include it because it would be installed anyway.
       nodejs
+      attic
     ];
 
     # To allow building on the host, must override the the service's config so it doesn't use a dynamic user