rustPackages: make rustc-dev optional

This was only introduced in 1.40.0 and doesn't work on older versions.

    thread 'main' panicked at 'Error: no rules matched rustc-dev.', src/bootstrap/builder.rs:231:21
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
    failed to run: /build/rustc-1.38.0-src/build/bootstrap/debug/bootstrap dist rustc-dev

pkgs/development/compilers/rust/default.nix

@@ -1,5 +1,6 @@
 { rustcVersion
 , rustcSha256
+, enableRustcDev ? true
 , bootstrapVersion
 , bootstrapHashes
 , selectRustPackage
@@ -70,6 +71,7 @@
       rustc = self.callPackage ./rustc.nix ({
         version = rustcVersion;
         sha256 = rustcSha256;
+        inherit enableRustcDev;
 
         # Use boot package set to break cycle
         rustPlatform = bootRustPlatform;