This removes the unpleasant failures of the dist job for tickets that add/update packages; and thus enables the local-macos-nohomebrew workflow for those.

The previous function of the dist job to check whether all SPKG tarballs have been uploaded is now taken over by a new standalone workflow (sdist.yml); it only runs on release tags.

CC: @mkoeppe @slel @DaveWitteMorris @kliem @dimpase

Component: scripts

Author: Frédéric Chapoton, Matthias Koeppe

Branch/Commit: 33e43d5

Reviewer: Matthias Koeppe, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/32403