add verbose shell script output option

We occasionally have challenging issues where hard-to-repro bugs occur in the gh workflows, such as in https://github.com/nerdvegas/rez/pull/1199.

Propose adding an env-var option for debugging purposes, that will dump any dynamically-generated shell code to stdout (such code is generated whenever a context is executed).

Start by tracing how a context is executed and where its dynamically generated shell code is produced. Check the gh workflow debugging context in pull request 1199 and existing environment-variable conventions; done means an opt-in environment variable sends generated shell code to stdout without changing normal output.

Written by the indexing model from the issue text.