ci: Disable parallel unit tests in CI

[acosta11]

Description

Port of recent update in other logging repos: allow CI to disable parallel tests to avoid flakes in resource constrained envs.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing performed?

• Unit tests
• Integration tests
• Acceptance tests

Checklist:

• This PR is being made against the main branch, or relevant version branch
• I have made corresponding changes to the documentation
• I have added testing for my changes

[ctlong]

The pipeline failure was unrelated. Unit tests are passing so this LGTM.