Mount rosdoc2 source directory read-only.

Our deprecated out-of-tree build option has been removed which means
that we now need to host our rosdoc2 sources in a read-write location to
allow build artifacts to be generated in-tree.

Mounting read-write shouldn't matter overmuch because we delete and
re-clone rosdoc2 each time the job runs[1].

[1]: https://github.com/ros-infrastructure/ros_buildfarm/blob/f8d8219b7b7566dcccd6b95a7cb880962cdd816e/ros_buildfarm/templates/doc/rosdoc2_job.xml.em#L94

ros_buildfarm/scripts/doc/build_rosdoc2.py

@@ -49,8 +49,7 @@ def main(argv=sys.argv[1:]):
  '-m',
  'pip',
  'install',
- '--no-warn-script-location',
- '--use-deprecated=out-of-tree-build',
+ '--break-system-packages',
  '.'],
  cwd=args.rosdoc2_dir)
  if pip_rc:

ros_buildfarm/templates/doc/rosdoc2_job.xml.em

@@ -146,7 +146,7 @@ else:
  ' --cidfile=$WORKSPACE/docker_generating_docker/docker.cid' +
  ' -e=HOME=/home/buildfarm' +
  ' -v $WORKSPACE/ros_buildfarm:/tmp/ros_buildfarm:ro' +
- ' -v $WORKSPACE/rosdoc2:/tmp/rosdoc2:ro' +
+ ' -v $WORKSPACE/rosdoc2:/tmp/rosdoc2' +
  ' -v $WORKSPACE/ws:/tmp/ws' +
  ' -v $WORKSPACE/docker_doc:/tmp/docker_doc' +
  hgcache_mount_arg +