fix: add flag to use runtime venv creation when using bootstrap=script (#2590)

The bootstrap=script implementation was changed to use declare_symlink()
to create explicit
symlinks so its venv works. Unfortunately, this broke packaging rules,
which would treat
the symlinks as regular files.

To fix, introduce a flag that stops using declare_symlink() and instead
creates the venv
at runtime. Creating a venv at runtime is problematic for various
reasons, but this should
work well enough until packaging rules are able to handle these raw
symlinks.

The location of the venv can be somewhat controlled by setting the
`RULES_PYTHON_VENVS_ROOT`
environment variable. This is to better accommodate cases where using
/tmp is problematic.

Along the way, sort the environment variable docs by their name.

Fixes https://github.com/bazelbuild/rules_python/issues/2489