venv: optional activate link/copy to solve ci/portability headaches

Problem

activate location is platform dependent.

Background

15+ years ago PyPY made a decision that has often been a headache for portability; putting activate in Scripts for Windows and bin for everyone else.

This is sometimes misreported as a bug (https://github.com/python/cpython/issues/97586), core issue (https://bugs.python.org/issue35003), discussion (https://github.com/python/cpython/pull/18083) and recurrent threads on the pypy issues list (https://github.com/pypa/virtualenv/issues/2404, https://github.com/pypa/virtualenv/issues/1418#issuecomment-910044412)

It's an exceptional pain point because 1/ you chose python to ease portability, -2->/ you use venv as an extra layer of abstraction, -3->/ surprise! your abstraction requires you to take two different routes based on platform!

Proposal

Introduce an option that causes the creation of a symlink or second copy of the activator at a user specified path.

Ubuntu|Pwsh> venv -p python 3.10 --activator-copy=/opt/ci/activate /opt/ci
Windows|Pwsh> venv -p python 3.10 --activator-copy=o:/ci/activate o:/ci

Arguments

(Per the original change) "Scripts is the correct location for activate on Windows"

• This doesn't move it, so everything that already supports the split will work as before,
• Python itself now violates the Scripts rule, which is an attractor to people who wanted to hide platform specifics,

It's not a big deal, just if .. then else

CI and building today is complex with many moving parts, having an intermediate level abstraction suddenly require you to think about platforms is always a headache. If you're using something like gitlab, you end up with a huge hitch in complexity to achieve this for the scope it's in under the current system.

Windows doesn't support symlinks

Whether it is a file or a symlink is irrelevant, and anyone who is actively working to support a version of Windows old enough to not support symlinks already has enough of their hair on fire that they'll gladly accept it falling back to creating a file instead of junction.

No files or tests are named. Trace venv creation and its command-line option handling, then verify the proposed user-specified activator path across Ubuntu and Windows PowerShell, including the symlink-or-copy behavior; done means the option works without changing existing platform-specific locations.