Internal: debuggging dockerfile cache misses

[martijnvans]

Description

During the build, dump a filesystem checksum view that should show us what things are different for github builds vs git-on-borg builds.

For more on the tool being installed, see https://github.com/MShekow/directory-checksum/tree/main?tab=readme-ov-file#directory-checksum.

Related issue

b/318538879

How has this been tested?

automated presubmits

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
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  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.