use flyctl from nixpkgs instead of manual install

.tangled/workflows/deploy.yaml

···
 
 dependencies:
   nixpkgs:
-    - curl
-    - cacert
-    - gnutar
-    - gzip
-    - bash
+    - flyctl
 
 steps:
-  - name: install flyctl
+  - name: verify flyctl
     command: |
-      curl -L https://github.com/superfly/flyctl/releases/latest/download/flyctl_Linux_x86_64.tar.gz -o flyctl.tar.gz
-      tar -xzf flyctl.tar.gz --no-same-owner
-      chmod +x flyctl
-      mv flyctl /usr/local/bin/
       flyctl version
 
   - name: deploy to fly.io