Fix wheel platform tag

[chris-allan]

If the installed Python is, for example, universal2, the default
platform tag always be universal2 even if the build is just
x86_64 like it is here. We need to fix the wheels we produce so
that we don't advertise platform tags that are not actually
supported.

See:

• arm64-only package selects universal2 as the arch tag on macOS 11 pypa/wheel#406

[chris-allan]

Not required with #4 as we are no longer targeting univeral2 compiled versions of Python on GitHub Actions.