--executable option to override sys.executable

[riastradh]

python -m installer supports several options for controlling the paths in the output when they don't match the Python system that is running:

• --destdir puts everything relative to a system root
• --prefix overrides sys.prefix for the installation prefix within that system root where files are installed

But there appears to be no way to control the path that is stored in #! lines of entry points that installer creates. In contrast, for example, historically with setuptools, python setup.py install would respect [build] executable = '/path/to/python' in setup.cfg; this functionality doesn't appear to be available through installer. It would be nice if python -m installer additionally supported:

• --executable overrides sys.executable for the Python executable path within that system root used in #! lines generated for entry points

This is important for cross-compilation into a fixed file system layout in the destdir that is independent of where Python happened to be installed on a development machine. For example, if I set up a virtualenv in /home/riastradh/venv for a cross-compilation toolchain, sys.executable will be /home/riastradh/venv/bin/python, but the scripts installed in the destdir need to start with #!/opt/pkg/bin/python -- just like files installed in the destdir need to go under /opt/pkg/lib/python3.12/site-packages according to --prefix, rather than under /home/riastradh/venv/lib/python3.12/site-packages according to sys.prefix.