gh-38111: GH Actions: Fix upload of macOS wheels to PyPI
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes #12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes #12345". -->
The change made in #37099 to accommodate the new design of
actions/{up,down}load-artifact@v4 broke the upload of macOS wheels to
PyPI (see https://github.com/sagemath/sage/actions/runs/9234698750/job/2
5408786962#step:8:23); this was masked so far because the build of macOS
wheels was broken until #36525.
Here we revert to using a separate job that uploads all platform wheels.
This uses the new keywords `pattern` and `merge-multiple` of
actions/download-artifact@v4
Example run:
https://github.com/mkoeppe/sage/actions/runs/9291646178/job/25572343853
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
- [x] The title is concise and informative.
- [x] 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.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
URL: https://github.com/sagemath/sage/pull/38111
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee
