Enable nightly feature for crossbeam-utils dep

Last two commits bumped rustc-ap-* crates which also transitively
updated rustc_data_structures. That crate enables the "nightly"
whereas Cargo's dep does not hence why we need to unify the features
to deduplicate the artifacts.

Cargo.lock

@@ -2733,6 +2733,7 @@ name = "rustc-workspace-hack"
 version = "1.0.0"
 dependencies = [
  "byteorder 1.2.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "crossbeam-utils 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "scopeguard 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)",

src/tools/rustc-workspace-hack/Cargo.toml

@@ -64,6 +64,7 @@ features = [
 
 [dependencies]
 curl-sys = { version = "0.4.13", features = ["http2", "libnghttp2-sys"], optional = true }
+crossbeam-utils = { version = "0.6.5", features = ["nightly"] }
 parking_lot = { version = "0.7", features = ['nightly'] }
 rand = { version = "0.6.1", features = ["i128_support"] }
 serde = { version = "1.0.82", features = ['derive'] }