blubber.yaml: Use latest buildkit stuff

Bump the blubber/buildkit frontend to v0.24.0.

Bump the version of buildkit from which buildctl is taken to
wmf-v0.14.1-3.

Made changes to comply with Blubber's new Python builder which always
creates a virtualenv.

Simplify the lint variant by not being picky about what local
context files are copied. Use an empty requirements list to avoid
needlessly installing packages that will be re-installed by tox.

Remove the chmod 0755 of /usr/local/bin/buildctl, which already has
the correct file mode in the the source image.

Prefix PATH with /srv/app/bin instead of attempting to make a symlink
to /srv/app/bin/kokkuri in /usr/local/bin (which doesn't work with the
new Blubber frontend).

Updated tests accordingly.