[cargo-zerocopy] Pass --cfg __ZEROCOPY_INTERNAL_USE_ONL_TOOLCHAIN=... (#2983)

This allows testing code to detect the current named toolchain. Unlike
the `rustversion` crate, it won't require us to update use sites when we
update pinned toolchain versions.

gherrit-pr-id: G9c6fe23f1983e0f1a3d30e1db70e020595260e23

Author: joshlf

tools/cargo-zerocopy/src/main.rs

@@ -169,6 +169,7 @@ fn get_rustflags(name: &str) -> String {
     let mut flags =
         "--cfg zerocopy_derive_union_into_bytes --cfg __ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE"
             .to_string();
+    flags += &format!(" --cfg __ZEROCOPY_INTERNAL_USE_ONLY_TOOLCHAIN=\"{name}\"");
 
     if name == "nightly" {
         flags += " --cfg __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS";