bazel: slim down build scripts

[ParkMyCar]

Previously I had thought build scripts could reference dependencies in the [dependencies] section of Cargo, but this is incorrect (docs)! This PR removes regular dependencies from build scripts which tidies things up a bit and should make the build faster.

Motivation

https://github.com/MaterializeInc/database-issues/issues/7923

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.

[ParkMyCar]

After this change the number of total build actions goes from 4,259 to 3,878! Because of remote caching there won't be too much of a speed up, but definitely will help in some cases