NotImplemented error with config env-var override is misleading

This gives a NotImplementedError that is not helpful:
```
]$ REZ_PACKAGE_FILTER='[]' rez-build
```
It should instead do something like this:
```
]$ REZ_PACKAGE_FILTER='[]' rez-build
Error: The "package_filter" setting does not support override via $REZ_PACKAGE_FILTER. You can try using REZ_PACKAGE_FILTER_JSON instead - you must set this to a json-compatible string.
```

Start by reproducing the REZ_PACKAGE_FILTER='[]' rez-build command, then trace the package_filter environment-variable override handling from the rez-build entry point. Done means the unsupported override produces a helpful error and suggests REZ_PACKAGE_FILTER_JSON with its JSON-string requirement.

Written by the indexing model from the issue text.