Now that the `org.osbuild.linux` runner does not use `api.setup_stdio`
anymore, the output of the binary run from the BuildRoot must end up
in `BuildRoot.output`. Check for that.
Create a new CompletedBuild object that wraps and is very similar
to the subprocess.CompletedProcess, i.e. it has a process member
but also has shortcuts for returncode. Additionally, the output
of the process is not only forwarded to the monitor, but also
captured and then handed to CompletedBuild, so its output member
will actually contain the full build output. To be compatible
with the previously returned CompletedProcess, `stderr`, `stdout`
members exist on CompletedBuild that also return `output`.
In case that bubblewrap fails to, e.g. because it fails to execute
the runner, it will print an error message to stderr. Currently,
this output is not capture and thus not logged. To fix that, the
`BuildRoot.run` method now takes a monitor object and will stream
stdout/stderr to the log via the monitor.
This is a crucial pre-condition for the org.osbuild.selinux stage
to work properly, especially that it can set labels that are not
present in the policy on the host. If /sys/fs/selinux is writable,
setfiles will try to verify the labels via /sys/fs/selinux/context
and fail for unknown labels.
