Update buildstream to latest stable release (1.6.2)

[Javier Jardón]

Hi!

Would it be possible to update buildstream to the latest stable
version (1.6.2) ?
The 1.4.x version is not really supported anymore and 1.6.x is needed
to build active projects like GNOME [2] or freedestkop-sdk [3]

Cheers,
Javier

[1] https://tracker.debian.org/pkg/buildstream
[2] https://gitlab.gnome.org/GNOME/gnome-build-meta/
[3] https://gitlab.com/freedesktop-sdk/freedesktop-sdk/