Suggest enabling patch-binaries-for-nix in shell.nix (rust-lang#1774)

Bootstraps nix detection isn't always perfect:
- https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Bootstrapping.20on.20NixOS
- rust-lang#115073

src/doc/rustc-dev-guide/src/building/suggested.md

@@ -282,6 +282,7 @@ let
     changelog-seen = 2
 
     [build]
+    patch-binaries-for-nix = true
     # The path to (or name of) the GDB executable to use. This is only used for
     # executing the debuginfo test suite.
     gdb = "${pkgs.gdb}/bin/gdb"