dont embed RUSTFLAGS in final binary

[BoxyUwU]

Change Summary

Fixes #1365

Using option_env! produces a static str literal for the value of the env var, this necessarily results in it being stored in the final binary. In this case the entirety of RUSTFLAGS would be present which can include paths which causes issues for reproducible builds.

I'm not sure if we have any way to really test for this though, it seems unfortunate to just fix stuff like this then wait until someone complains if we break something 😅

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1396 will not alter performance

_{Comparing boxy/dont_embed_rustflags (add996a) with main (6e96b85)}

Summary

✅ 155 untouched benchmarks

[davidhewitt]

Thanks!

[moto-timo]

Thank you!

[moto-timo]

I agree about test cases... not sure how to address that in e.g.. the GitHub workflow, but we do reproducible build testing in Yocto Project.

[davidhewitt]

Have you got a standard pattern for testing reproducible builds in Yocto that we might be able to base something off?

[moto-timo]

https://docs.yoctoproject.org/test-manual/reproducible-builds.html

TL;DR
We build with two different build paths and compare the two with diffoscope.
In our case:

1. the first build uses "shared state" which is cached binaries from the tasks already run
2. the second build is from scratch with no prior caching.

[moto-timo]

Related rust-lang/cargo#5505