chore: Replace build.rs with lint config in Cargo.toml (#1354)

Rust 1.80 [just
dropped](https://blog.rust-lang.org/2024/07/25/Rust-1.80.0.html), adding
a lint option that allows us to avoid the `build.rs` definition we had
to add for #1067.
(The nightly lint got updated before hitting beta, so there shouldn't be
compatibility problems there).

~~Depends on #1355~~

Cargo.toml

@@ -14,8 +14,9 @@ license = "Apache-2.0"
 # authors
 
 [workspace.lints.rust]
-missing_docs = "warn"
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(ci_run)'] }
 
+missing_docs = "warn"
 [workspace.lints.clippy]
 # Unstable check, may cause false positives.
 # https://github.com/rust-lang/rust-clippy/issues/5112