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/cli/secrets_test.rb

@@ -3,7 +3,7 @@ require_relative "cli_test_case"
 class CliSecretsTest < CliTestCase
   test "fetch" do
     assert_equal \
-      "\\{\\\"foo\\\":\\\"oof\\\",\\\"bar\\\":\\\"rab\\\",\\\"baz\\\":\\\"zab\\\"\\}",
+      "{\"foo\":\"oof\",\"bar\":\"rab\",\"baz\":\"zab\"}",
       run_command("fetch", "foo", "bar", "baz", "--account", "myaccount", "--adapter", "test")
   end