Escape secrets in inline command substitution

Kamal "inlines" calls to `kamal secrets` in the dotenv file, but the
results of the calls were not being escaped properly. To "fix" this
`kamal secrets fetch` escaped the JSON string before returning it.

The two errors cancelled out, but it meant that the commands didn't
work from a shell.

To fix, we'll escape the inline command results and remove the escaping
from `kamal secrets fetch`.

test/test_helper.rb

@@ -86,8 +86,4 @@ class SecretAdapterTestCase < ActiveSupport::TestCase
       stub_ticks.with { |c| c == command && (succeed ? `true` : `false`) }
       Kamal::Secrets::Adapters::Base.any_instance.stubs(:`)
     end
-
-    def shellunescape(string)
-      "\"#{string}\"".undump.gsub(/\\([{}])/, "\\1")
-    end
 end