chore: bump nightly version used in CI (#2178)

This requires fixing a few warnings.

tokio-macros/src/lib.rs

@@ -14,6 +14,9 @@
 
 //! Macros for use with Tokio
 
+// This `extern` is required for older `rustc` versions but newer `rustc`
+// versions warn about the unused `extern crate`.
+#[allow(unused_extern_crates)]
 extern crate proc_macro;
 
 mod entry;