Fix remote builds by making build-config contents deterministic

[cbarrete]

See the commit message for context.

Note that it I am not 100% confident about what impact this has on cross compilation, because there are no tests/examples for it.

[cbarrete]

Hi @davidhewitt, I hope that you don't mind the ping, I just wanted to raise that in case you had missed it.

[Tpt]

The commit message for reference:

Because of the usage of env!, we were baking paths to generated files
in lib.rs. In setups where the build can be executed remotely (e.g.
when building with Buck2), those paths would be different depending on
whether the build ran locally or remotely.

This difference propagates to pyo3-macros. Because Buck2 runs the
metadata and codegen builds separately (for increased parallelism), they
can have different contents due to the above, causing rustc to find two
versions of the crate, and erroring out. This is not observable with
Cargo because it runs both builds with the same rustc invocation.

See facebook/buck2#1206 and
rwf2/Rocket#2797 for more context.

This commit fixes the issue by simply reading OUT_DIR at run-time,
rather than compile-time, so that the content of lib.rs is fixed.

Closes facebook/buck2#1206

Many thanks to https://github.com/cormacrelf for pointing out the root
of the issue.

[Tpt]

Make sense to me. Waiting to @davidhewitt before merging. Thank you for this!

[davidhewitt]

Thanks for the PR, and sorry to have been so slow to reply.

Sadly almost this exact patch has been proposed before, and I still think unfortunately it will have problems with cross-compilation. Please see #4579 which has my notes from the issue last time it was discussed.

[cbarrete]

I see, thanks for the link, I didn't find it when looking for this issue.
I'll close this PR and comment there.