Pin goopack version

[jefferbrecht]

Description

The latest version of goopack (v2.20.0) has an issue with path separators, so pin to the newest working version instead (v2.18.4).

See google/googet#83 (comment).

Related issue

google/googet#83 (comment)
b/383588824

How has this been tested?

Verified manually.

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • 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.