message: buck2: update Cargo builds to nightly 2021-10-22
diff: #![feature(const_fn_trait_bound)]
  #![feature(const_intrinsic_copy)]
  #![feature(const_mut_refs)]
  -#![feature(const_panic)]
  #![feature(const_ptr_offset)]
  #![feature(const_raw_ptr_deref)]
  #![feature(const_refs_to_cell)]
  #![feature(thread_local)]
  #![feature(try_blocks)]
  #![feature(iter_intersperse)]
  +#![cfg_attr(fbcode_build, feature(const_panic))]
  //
  // Plugins
  #![cfg_attr(feature = "custom_linter", feature(plugin))]
  