_sandboxbuildboxrun.py: Support host-files
This adds support for bst shell
with host-files to the buildbox-run sandboxing backend. This feature requires buildbox-run-bubblewrap with bind-mount support: BuildGrid/buildbox/buildbox-run-bubblewrap!4 (merged)
buildbox-run --capabilities
is used to determine whether bind-mount support is available. If it's not available, the sandbox prints a warning.
This MR fixes 6 xfails with buildbox-run.
Part of #719 (closed).