fix: Ensure SSH connections are closed after each command execution

This commit addresses an issue where SSH connections were not being properly closed after each command execution, leading to timeout errors during the `kamal deploy` process.

lib/kamal/sshkit_with_ext.rb

@@ -118,6 +118,8 @@ def execute
         rescue ::StandardError => e
           e2 = SSHKit::Runner::ExecuteError.new e
           raise e2, "Exception while executing #{host.user ? "as #{host.user}@" : "on host "}#{host}: #{e.message}"
+        ensure
+          SSHKit::Backend::Netssh.pool.close_connections
         end
       end