Fix FrozenError when using interactive environment selector

[mattbrictson]

When running tomo deploy in a multi-environment project with no environment explicitly specified, tomo prompts:

Choose an environment: 
❯ staging
  production

Interacting with this menu with the keyboard would result in an error:

forwardable.rb:240:in 'String#clear': can't modify frozen String: "" (FrozenError)
	from forwardable.rb:240:in 'Tomo::Console::KeyReader#clear'

This is because the internal buffer used by KeyReader was frozen as a side effect of #446.

Fix by marking buffer as mutable.