The system_python bootstrap is basically just an alternative stage1
bootstrap now.
Finish unifying it with bootstrap=script by having it support and create
a full venv.

While this is the new default behavior, it's disabled for:
* Windows: some difficult to debug failures came up, so disable using it
there
* Bazel 7: Bazel 7 doesn't have the `File.is_symlink` APIs, so it's
  problematic to enable it there due to zipapp support.

Switching from non-venv to venv layouts is a significant change and
likely to
break something, so marking this as a breaking change.

Additional notes:
* rules_pkg 1.2+ is needed to package a venv-based binary, due to
extensive use
  of symlinks.

Work towards #2156