When using modules for compiler (and/or external package), if a
package's `setup_[dependent_]build_environment` sets `PYTHONHOME`, it
can influence the python subprocess executed to gather module
information.
The error seen was:
```
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```
But the actual hidden error happened in the `python -c 'import
json...'` subprocess, which made it return an empty string as json:
```
ModuleNotFoundError: No module named 'encodings'
```
This fix uses `python -E` to ignore `PYTHONHOME` and
`PYTHONPATH`. Should be safe here because the python subprocess code
only use packages built-in python.
The python subprocess in `environment.py` was also patched to be safe
and consistent.
0d6a2381b2