Set KAMAL_DESTINATION when loading config

[nickhammond]

[jeromedalbert]

I'm glad to see that this issue is being tackled. I have needed ENV['KAMAL_DESTINATION'] in my config/deploy.yml so I can do Dotenv.load(".kamal/secrets.#{ENV['KAMAL_DESTINATION']}") or something similar with Dotenv.parse.

This PR would remove my hacky workaround of pre-parsing the destination from ARGV in my bin/kamal binstub and setting the environment variable then.