Executor probing should use a multiarch-compatible rootfs image

[staticfloat]

We need to not just blithely use alpine_rootfs(), but rather use a muilti-arch compatible rootfs image when doing things like probing for executor compatibility.

[DilumAluthge]

From Elliot on Slack, the workaround until this issue is solved is:

the intermediate fix is you define FORCE_SANDBOX_MODE=unprivileged in your environment to short-circuit the check