environment subcommand: flag to NOT inherited `PYTHONPATH`

## Summary

When `cyclonedx-py environment ` is invoked from a process that has
`PYTHONPATH` set, the `PYTHONPATH` is inherited by the subprocess that
`cyclonedx-py` spawns to enumerate `sys.path` of the target interpreter. As a
result, packages from outside the target venv appear in the generated SBOM -
packages that are not installed in the environment being scanned at all.

## Root cause (probably)

In cyclonedx_py/_internal/environment.py, the __path4python method spawns a
subprocess to read the target interpreter's `sys.path`:

```python
# cyclonedx_py/_internal/environment.py, __path4python()
res = run(cmd, capture_output=True, encoding='utf8', shell=False) # nosec
```

`subprocess.run` is called without an explicit env= argument, so it inherits the
full environment of the calling process including `PYTHONPATH`. When
`PYTHONPATH` is set in the caller (e.g. by a build tool, test runner, or task
runner that places its own packages there), those paths are prepended to
`sys.path` inside the spawned interpreter. The resulting path list is then used
by `importlib.metadata.distributions(path=...)` to discover packages, so any
package reachable via the inherited `PYTHONPATH` is included in the SBOM as if
it were installed in the target venv.

## Minimal reproducible example

```python
uv tool install cyclonedx-bom

# Create the venv to be scanned (install only 'requests')
uv venv /tmp/scan_venv
uv pip install --python=/tmp/scan_venv/bin/python requests

# Create a separate directory with an unrelated package visible via PYTHONPATH
uv venv /tmp/outer_venv
uv pip install --python=/tmp/outer_venv/bin/python flask

# Run cyclonedx-py with PYTHONPATH pointing at the outer package directory
PYTHONPATH=$(/tmp/outer_venv/bin/python -c "import sysconfig; print(sysconfig.get_path('purelib'))") \
cyclonedx-py environment /tmp/scan_venv -o /tmp/sbom.json

# Inspect the result -> 'Flask' (and its dependencies) appear in the SBOM
# even though it is NOT installed in /tmp/scan_venv
uv run python -c "
import json
data = json.load(open('/tmp/sbom.json'))
names = [c['name'] for c in data.get('components', [])]
print('flask in SBOM:', 'Flask' in names)
print('All components:', sorted(names))
"
```

Expected: SBOM contains only requests (and its dependencies).

Actual: SBOM also contains flask (and its dependencies), which were never
installed in the scanned venv.

## Why it matters

Any tool that invokes cyclonedx-py in-process or as a subprocess after setting
PYTHONPATH including build systems, task runners, and CI pipelines that add
their own directories to PYTHONPATH will silently produce an inaccurate
SBOM. The SBOM over-reports dependencies that are not actually present in the
scanned environment, which undermines the entire purpose of SBOM generation for
supply-chain auditing and compliance.

## Suggested fix

Strip PYTHONPATH (and, for robustness, VIRTUAL_ENV and PYTHONHOME) from the
environment passed to the subprocess in `__path4python`:

```python
import os

def __path4python(self, python: str, import_site: bool) -> list[str]:
cmd = [self.__py_interpreter(python),
'-c', 'import json,sys;json.dump(sys.path,sys.stdout)']
if not import_site:
cmd.insert(1, '-S')

# Explicitly clear env vars that would pollute the target interpreter's
# sys.path and cause packages from outside the target venv to appear in
# the discovered distributions.
clean_env = {
k: v for k, v in os.environ.items()
if k not in ('PYTHONPATH', 'PYTHONHOME', 'VIRTUAL_ENV')
}

self._logger.debug('fetch `path` from python interpreter cmd: %r', cmd)
res = run(cmd, capture_output=True, encoding='utf8', shell=False, env=clean_env)
...
```

## Environment

- Package version: cyclonedx-bom 7.3.0
- Python version: 3.10+
- Platform: Ubuntu 24.04

## Contribution

- [x] I am willing to provide a fix
- [ ] I will wait until somebody else fixes it

Read cyclonedx_py/_internal/environment.py, focusing on __path4python and its subprocess invocation. Run the minimal PYTHONPATH reproduction described in the issue, then verify that the generated SBOM contains only packages installed in the scanned virtual environment and excludes packages exposed by the caller's environment.

索引モデルが issue の本文から書いたものです。