Tagging 1.91.0 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 'juerg/artifactshare' into 'master'

tests/testutils/artifactshare.py: Add SIGTERM handler to subprocess

See merge request BuildStream/buildstream!1606