[CI] Separate Python wheel upload jobs for MacOS and Linux.

[mikeurbach]

A while back, we separated the wheel build and wheel upload jobs, since the wheel upload we use only works in a Linux environment. However, we sometimes see failures on one platform that block the upload even if the other platform succeeded.

As mentioned in #6385 (comment), the recommended way to allow jobs to proceed even if other jobs fail doesn't seem to work for our scenario.

To resolve this, I think we could separate the wheel upload jobs for MacOS and Linux, so that even if the build fails on one platform, the other platform can be uploaded if it succeeds. This might also require separating the build wheel jobs by platform, so each upload wheel job can depend on the build wheel job for its platform.