fix(flake.nix): specify tokio_unstable cfg (#942)

Summary:
This is supposed to be specified in .cargo/config.toml but since we add the environment variable, it overrides that.

Fixes: #941

Pull Request resolved: #942

Reviewed By: alexlian

Differential Revision: D74213872

fbshipit-source-id: eef95f1a37fd69a3f40895a1388e1e7085c62769

Author: lf-

flake.nix

@@ -38,14 +38,16 @@
           Security
         ]);
         packages = [ pkgs.cargo-bloat my-rust-bin pkgs.mold-wrapped pkgs.reindeer pkgs.lld_16 pkgs.clang_16 ];
-        shellHook = 
+        shellHook =
           ''
             export BUCK2_BUILD_PROTOC=${pkgs.protobuf}/bin/protoc
             export BUCK2_BUILD_PROTOC_INCLUDE=${pkgs.protobuf}/include
           ''
           # enable mold for linux users, for more tolerable link times
+          # we have to specify tokio_unstable in the RUSTFLAGS here since they override
+          # .cargo/config.toml that is the reasonable place to specify it
           + pkgs.lib.optionalString pkgs.stdenv.isLinux ''
-            export RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold $RUSTFLAGS"
+            export RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold --cfg=tokio_unstable $RUSTFLAGS"
           '';
       };
     });