run-bubblewrap: Fix `/tmp` tmpfs condition

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

If not required, please explain in brief why not.

Description

tmpfs should be used if there are neither /tmp inputs nor /tmp outputs.

This also fixes the /tmp output test cases. Using / affects the behavior depending on the host environment. Use a controlled input root instead.

Validation

The test suite has been updated.