Add a `force-sdist` input to the Python release workflow.

The first run for this month's release failed after uploading some
of the wheels, but before uploading the sdist, so we need a way to
force the sdist on a rerun if necessary.

Change-Id: I3851d20cdeba02caad5f450e77cd7ef8704990d2
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63170
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>