Installation guide: Do not recommend --prefix=$CONDA_PREFIX

[mkoeppe]

Sage packages can clobber installed non-Python conda packages.
Instead we change it to recommend --with-sage-venv=$CONDA_PREFIX and to put SAGE_LOCAL in a subdirectory of the CONDA_PREFIX.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 45e2591; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[kwankyu]

While we are at it, would you unindent the surrounding block (which is shown as quoted text, unintentionally I think) here and below?

[kwankyu]

Sorry but would you explain how "clobber" happens and why the change in this PR solves the problem?

[kwankyu]

Now

                    --with-sage-venv=$CONDA_PREFIX 

seems reasonable to me.

Is there an argument to prefer

                    --prefix=$CONDA_PREFIX/libexec/sage-local

than other choices, such as $CONDA_PREFIX/share/sage/local?

[kwankyu]

Sorry but would you explain how "clobber" happens and why the change in this PR solves the problem?

I think I can answer this. Previously sage installs files into $CONDA_PREFIX/share, and these files may conflict with files that conda itself manages.

$CONDA_PREFIX/share/sage/local ?

Then this also could be problematic...

[kwankyu]

We may just create a new directory instead of treading into conda areas. How about

--prefix=$CONDA_PREFIX/sage/local

?