[Services] Cleanup commtest/build.rs

Summary:
Apply feedback from https://phab.comm.dev/D5102?id=16560#inline-33198.

Also:

• Groom the file to be more idiomatic rust.
• Ask cargo to rebuid if the protobuf files change.

D5102 was "how little do I need to do to make it build". Where as this revision
is to be a more holistic, "how would I write this file".

Test Plan:

cd services/commtest
cargo build

Reviewers: varun, tomek, marcin

Reviewed By: varun

Subscribers: ashoat, karol, abosh

Differential Revision: https://phab.comm.dev/D5154