Add rustc explicitly to flake (TraceMachina#398)

The previous implementation would automatically fetch rustc for cargo
builds. Explicitly add rustc to the flake so that it's version is pinned
and in sync with the cargo package.

flake.nix

@@ -25,6 +25,7 @@
             nativeBuildInputs = [
               # Development tooling goes here.
               pkgs.cargo
+              pkgs.rustc
               openssl_static # Required explicitly for cargo test support.
               bazel
             ];