Prevent new lint from turning off nightly span support under -Dwarnings

Author: dtolnay

build.rs

@@ -156,6 +156,7 @@ fn compile_probe(rustc_bootstrap: bool) -> bool {
         .arg("--edition=2021")
         .arg("--crate-name=proc_macro2")
         .arg("--crate-type=lib")
+        .arg("--cap-lints=allow")
         .arg("--emit=dep-info,metadata")
         .arg("--out-dir")
         .arg(out_dir)