Ensure instance cleanup runs even when context is cancelled

[sarahhodne]

We use context cancellations for things like shutdown with SIGTERM and timeouts, but some log messages seem to indicate that this could mean that the instance shutdown we do to clean up the instance after a cancellation doesn't finish. We should ensure that a context cancellation would still allow this cleanup to run.

Backends to check:

• cloudbrain
• docker
• gce
• jupiterbrain
• local (this might not apply to this backend)