using stage configuration variables in capistrano append

deploy.rb

# abuse capistrano variable validators to
# use stage configs with the non-lazy #append

# this bit sets a global config before the stage is loaded
append :linked_files, 'config/database.yml', 'config/other.yml'

# this bit will run after the variable you need is set
validate :rails_env do |_, value|
  append :linked_files, "config/credentials/#{value}.key"
end