flake: Include --cfg tokio_unstable in Rust flags

The flake sets `RUSTFLAGS` and that overwrites the setting in
`.cargo/config.toml`. We need to add the `--cfg tokio_unstable` flag to
enable integration tests to run when called from the devShell.

flake.nix

@@ -114,7 +114,7 @@
         if pkgs.stdenv.isLinux
         then pkgs.stdenv
         else pkgs.clangStdenv;
-      rustFlagsEnv = pkgs.lib.optionalString stdenv.isLinux "-C link-arg=-fuse-ld=lld -C target-cpu=native -Clink-arg=-Wl,--no-rosegment";
+      rustFlagsEnv = pkgs.lib.optionalString stdenv.isLinux "-C link-arg=-fuse-ld=lld -C target-cpu=native -Clink-arg=-Wl,--no-rosegment --cfg tokio_unstable";
       rustToolchain = pkgs.pkgsBuildHost.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
       craneLibMSRV = (crane.mkLib pkgs).overrideToolchain rustToolchain;
       craneLibStable = (crane.mkLib pkgs).overrideToolchain pkgs.pkgsBuildHost.rust-bin.stable.latest.default;