feat: add runner to build configuration

[baszalmstra]

The Runner API now exists in rattler_build_script, but the build configuration has no place to carry a runner. Library integrations that already pass Configuration through the build and test code would otherwise need to keep the execution backend as separate state.

This adds an Arc<dyn Runner> to Configuration and a with_runner builder method. The default stays LocalRunner, so existing callers keep the current behavior. CLI builds construct the local runner from the selected environment isolation mode, keeping those settings together. The runner types are also re-exported through the core script API.

This is configuration and API plumbing only. Existing script execution is unchanged.

Testing

• pixi run cargo-fmt-check
• pixi run cargo check --workspace --all-targets
• pixi run cargo clippy -p rattler_build_core --all-targets --all-features -- -D warnings -Dclippy::dbg_macro
• pixi run cargo check --manifest-path py-rattler-build/rust/Cargo.toml
• pixi run test

[wolfv]

Not sure if these tests add a lot of value lol