fix(tonic-prost-build): restore missing emit_rerun_if_changed functionality

[pierrekin]

Motivation

Solves issue where emit_rerun_if_changed is ignored.

Based on a cursory analysis it seems emit_rerun_if_changed was broken since the tonic-prost-build extraction:

• June 21, 2022 (commit 1d2083a): emit_rerun_if_changed feature added to tonic-build with working implementation
• July 25, 2025 (commit 969408e): tonic-prost-build extracted from tonic-build - implementation accidentally omitted

Original implementation: 1d2083a#diff-b0934b805d6c8c11cd87db5e7e77d3e5d2d72d5c72d36e16ce7e99516a8e6b84R435-R449

Extraction commit that lost the implementation: 969408e

Solution

Restores the missing implementation that emits cargo:rerun-if-changed directives for proto files.

[pierrekin]

After reviewing the code more carefully, I believe it was the intention of the original author to move the implementation to the prost-build. I will make a subsequent commit that implements this, but it will need to wait for an upstream change to prost-build.

[pierrekin]

A note to reviewers:

After reviewing the code more carefully, I believe it was the intention of the original author to move the implementation to prost-build.

feat(prost-build): add emit-rerun-if-changed functionality #1402

If my PR is accepted by Tokio, then I think we can continue along that route, if not, we should consider emitting the directives here as per my previous commit:

dea524c