Support a NOP run mode beyond virtual chalking

## Description

There are cases where for whatever reason a chalk bug could be affecting the build or execution of an image / binary etc. While we can do our best to safeguard against errors (see #188 and similar issues), we should offer a well-tested, robust `nop` mode of execution, where chalk only passes execution to the underlying command (e.g., if doing `chalk docker build ` to just do `docker build `). This will be different than standard chalk operations like virtual chalking, in the sense that there will be

* no con4m related initialization or code executing
* no requirement for setup/env etc.
* no chalk flag like `trace` etc. respected

This will offer an easy escape path for users in case of critical bugs, as well as a quick debug flag to pinpoint whether an issue is related to chalk's side effects vs not.

## Result

Define an environment variable (e.g., `__CHALK_NOP`), which, when set, will result in a completely separate path of execution, which is dependency free, and only calls execve with the underlying arguments passed to chalk.