Tagging 1.91.1 version.

This is not a specific release tag, but simply serves to ensure
that BuildStream master will report an appropriate version number
in the 1.91 range leading up to 2.0.
Merge branch 'bschubert/1.91.1' into 'master'

NEWS: update release notes for version 1.91.1

See merge request BuildStream/buildstream!1646