build: remove WORKSPACE from ci-builder calculation

[ParkMyCar]

I don't think anything in the WORKSPACE file should impact what gets included in the ci-builder image, and when we do include it, small changes like adding a new crate result in re-building the image.

@def- can you remember why we made this change? It wasn't immediately obvious to me from the original PR #30674.

Motivation

Less frequent rebuilds of the CI builder image.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.