[BuildStream] Switching to buildbox-run for local builds

[Jürg Billeter <j bitron ch>]

Hi all,

We've been working towards buildbox-run as sandboxing backend for local
builds[1] for a while. Feature parity with the integrated bwrap backend
on Linux has been reached with buildbox-run-bubblewrap and buildbox-
fuse. Test builds of a bst2 branch of freedesktop-sdk have been
successful. It's time to switch BuildStream master to buildbox-run as
primary sandboxing backend. The installation instructions are being
updated[2].

Right now with BuildStream master, buildbox-run is used only if
explicitly requested with BST_FORCE_SANDBOX=buildbox-run. I'm planning
to submit an MR to default to buildbox-run as soon as the pending MRs
have landed in BuildStream[3] and bst-plugins-experimental.

Shall we immediately drop the bwrap backend or shall we have a period
where the bwrap backend is still available with BST_FORCE_BACKEND=bwrap
in case we discover temporary issues? I don't think such parallel
support is crucial as, in the case of a blocking issue, users could
switch back to an older master commit until the issue has been
resolved. However, I also don't see a problem keeping the bwrap backend
around for a short while and dropping the related code in a few weeks
if that would ease the transition.

Any users of BuildStream master are welcome to give it a try right now,
of course, by setting BST_FORCE_SANDBOX=buildbox-run. Let me know if
you encounter any issues.

Cheers,
Jürg

[1] https://gitlab.com/BuildStream/buildstream/issues/719
[2] https://gitlab.com/BuildStream/website/-/merge_requests/135
[3] https://gitlab.com/BuildStream/buildstream/-/merge_requests/1887