Fixup sdist release. (#5067)
Fixup sdist release. (#5067)

Previously the publishing step assumed sdists had been generated but
there was no step actually doing this. Have `publish_packages` call
`build_packages` to ensure this.

Fixes #5027
foursquare/pants,benjyw/pants,UnrememberMe/pants,fkorotkov/pants,wisechengyi/pants,twitter/pants,pantsbuild/pants,wisechengyi/pants,benjyw/pants,wisechengyi/pants,baroquebobcat/pants,tdyas/pants,twitter/pants,wisechengyi/pants,foursquare/pants,twitter/pants,fkorotkov/pants,UnrememberMe/pants,pantsbuild/pants,wisechengyi/pants,UnrememberMe/pants,tdyas/pants,baroquebobcat/pants,UnrememberMe/pants,foursquare/pants,twitter/pants,UnrememberMe/pants,wisechengyi/pants,tdyas/pants,baroquebobcat/pants,foursquare/pants,UnrememberMe/pants,twitter/pants,fkorotkov/pants,fkorotkov/pants,benjyw/pants,wisechengyi/pants,benjyw/pants,twitter/pants,jsirois/pants,tdyas/pants,baroquebobcat/pants,tdyas/pants,baroquebobcat/pants,fkorotkov/pants,twitter/pants,jsirois/pants,twitter/pants,fkorotkov/pants,baroquebobcat/pants,fkorotkov/pants,baroquebobcat/pants,UnrememberMe/pants,tdyas/pants,foursquare/pants,benjyw/pants,benjyw/pants,fkorotkov/pants,tdyas/pants,baroquebobcat/pants,baroquebobcat/pants,pantsbuild/pants,benjyw/pants,UnrememberMe/pants,tdyas/pants,wisechengyi/pants,tdyas/pants,pantsbuild/pants,pantsbuild/pants,twitter/pants,wisechengyi/pants,UnrememberMe/pants,foursquare/pants,foursquare/pants,pantsbuild/pants,pantsbuild/pants,foursquare/pants,jsirois/pants,foursquare/pants