fix(root): run private package changed tests

[zahin-mohammad]

Description

• Pass Lerna's --private filter to unit-test-changed.
• Recreate the closed Ralph PR with a Conventional Commit that satisfies the repository's 100-character commit-body limit.

Issue Number

WAL-1960

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• yarn commitlint --from=HEAD~ --to=HEAD -V
• yarn postinstall
• yarn lerna run --scope bitgo unit-test --private --stream reaches the bitgo suite; completing that suite locally requires BITGOJS_TEST_PASSWORD, which CI injects.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My commits follow Conventional Commits and include the ticket reference
• New and existing unit tests pass locally with my changes (blocked on BITGOJS_TEST_PASSWORD)

[linear-code]

WAL-1960