fix memchr features in workspace-hack

Cargo.lock

@@ -3525,6 +3525,7 @@ dependencies = [
  "crossbeam-utils",
  "libc",
  "libz-sys",
+ "memchr",
  "proc-macro2",
  "quote",
  "rand_core 0.5.1",

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

@@ -79,6 +79,10 @@ crossbeam-utils = { version = "0.8.0", features = ["nightly"] }
 libc = { version = "0.2.79", features = ["align"] }
 # Ensure default features of libz-sys, which are disabled in some scenarios.
 libz-sys = { version = "1.1.2" }
+
+# looks like the only user of deprecated `use_std` feature is `combine`, so this
+# can be removed if/when https://github.com/Marwes/combine/pull/348 be merged and released.
+memchr = { version = "2.5", features = ["std", "use_std"] }
 # same for regex
 regex = { version = "1.5.6" }
 proc-macro2 = { version = "1", features = ["default"] }