fix: make sys.executable work with script bootstrap

[rickeylev]

When --bootstrap_impl=script is used, PYTHONPATH is no longer used to set the import
paths, which means subprocesses no longer inherit the Bazel paths. This is generally a
good thing, but breaks when sys.executable is used to directly invoke the interpreter.
Such an invocation assumes the interpreter will have the same packages available and works
with the system_python bootstrap.

To fix, have the script bootstrap use a virtual env. This allows it to intercept interpreter
startup even when the Bazel executable isn't invoked.

DO NOT MERGE:

• Update VERSION_NEXT markers
• Remove ## markers from stage2 bootstrap

Fixes #2169