require missing net/scp dependency

[juan-apa]

Preface

A couple of days ago, the github actions I have that automate the deployment of rails apps with kamal, started failing when running kamal envify, with the following error:

<internal:/Users/juan/.rbenv/versions/3.2.2/lib/ruby/3.2.0/rubygems/core_ext/kernel_require.rb>:37:in `require': cannot load such file -- net/scp (LoadError)
  ...

I tracked down the issue to one of the most recent releases of capistrano/sshkit, which removed require 'net/scp'
The capistrano release which no longer requires net/scp is v1.22.0 introduced in capistrano/sshkit#524

Steps to reproduce

• install kamal on a fresh ruby environment
• run any command which makes use of scp functionality (e.g. kamal envify)

The fix

• adding the require explicitly in lib/kamal/sshkit_with_ext.rb brings back the scp functionality.

I believe this is very similar to #635 which @mdkent did 2 weeks ago

This change fixes #644

[djmb]

Thank's for the PR 🙏