This adds a `--path` option to `spack python` that shows the `python`
interpreter that Spack is using.
e.g.:
```console
$ spack python --path
/Users/gamblin2/src/spack/var/spack/environments/default/.spack-env/view/bin/python
```
This is useful for debugging, and we can ask users to run it to
understand what python Spack is picking up via preferences in `bin/spack`
and via the `SPACK_PYTHON` environment variable introduced in #21222.
8d3272f82d