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/secrets/dotenv_inline_command_substitution_test.rb

@@ -12,4 +12,10 @@ class SecretsInlineCommandSubstitution < SecretAdapterTestCase
     substituted = Kamal::Secrets::Dotenv::InlineCommandSubstitution.call("FOO=$(blah)", nil, overwrite: false)
     assert_equal "FOO=results", substituted
   end
+
+  test "escapes correctly" do
+    Kamal::Cli::Main.expects(:start).with { |command| command == [ "secrets", "fetch", "...", "--inline" ] }.returns("{ \"foo\" : \"bar\" }")
+    substituted = Kamal::Secrets::Dotenv::InlineCommandSubstitution.call("SECRETS=$(kamal secrets fetch ...)", nil, overwrite: false)
+    assert_equal "SECRETS=\\{\\ \\\"foo\\\"\\ :\\ \\\"bar\\\"\\ \\}", substituted
+  end
 end