Hide secrets in Studio plug debugger results

The plug debugger in Studio now gives a plug the procedure's environment variables, secrets included, so a method could return a secret and show it in clear.

• Fixed secret values appearing in plug debugger results and errors: they are now masked, as they already are in tofupilot plug call output and plug logs.
• Added an end-to-end check that the procedure's env: reaches a debugged plug, with the plug's own env: taking precedence.