fix(ci): Replace busybox with ubuntu to avoid "device or resource busy" failures

[teor2345]

Motivation

Some CI jobs are regularly failing with "device or resource busy" errors.

Close #7659

Solution

The busybox image is really old, so I replaced it with a newer ubuntu image.

This could have been a kernel/OS incompatibility. Or this PR might fix the issue because it slows down automatic checks on the disks, allowing the mount command to win the race.

Review

This is urgent so I'm going to admin-merge it if it works. It needs to admin-merge so the checkpoint rebuild job can run.

We need to run it at least 3 times to test that it works.

Reviewer Checklist

• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

Follow Up Work

Work out if we can use ubuntu-slim instead to save image download & launch time.

[teor2345]

These runs were successful, except for the known issue with the checkpoint rebuild:

• https://github.com/ZcashFoundation/zebra/actions/runs/6425320111/attempts/1?pr=7686
• https://github.com/ZcashFoundation/zebra/actions/runs/6425320111/attempts/2?pr=7686 (just the checkpoint job)
• https://github.com/ZcashFoundation/zebra/actions/runs/6425320111/attempts/3?pr=7686
  • there was one device busy failure here
• https://github.com/ZcashFoundation/zebra/actions/runs/6425320111/attempts/4?pr=7686
• https://github.com/ZcashFoundation/zebra/actions/runs/6425320111/attempts/5?pr=7686
  • there was one unrelated failure here

This seems to be better than the current main branch, so maybe it's worth merging this change.