Fix JSONDecodeError when using compiler modules (#25624)

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.

lib/spack/spack/util/environment.py

@@ -989,7 +989,7 @@ def _source_single_file(file_and_args, environment):
         python_cmd = python_cmd.name if python_cmd else sys.executable
 
         dump_cmd = 'import os, json; print(json.dumps(dict(os.environ)))'
-        dump_environment = python_cmd + ' -c "{0}"'.format(dump_cmd)
+        dump_environment = python_cmd + ' -E -c "{0}"'.format(dump_cmd)
 
         # Try to source the file
         source_file_arguments = ' '.join([

lib/spack/spack/util/module_cmd.py

@@ -47,7 +47,7 @@ def module(*args):
         module_cmd += 'LD_LIBRARY_PATH="$SPACK_LD_LIBRARY_PATH" '
 
         # Execute the python command
-        module_cmd += '%s -c "%s";' % (sys.executable, py_cmd)
+        module_cmd += '%s -E -c "%s";' % (sys.executable, py_cmd)
 
         # If LD_LIBRARY_PATH was set after `module`, dump the old value because
         # we have since corrupted it to ensure python would run.