Make hardened_malloc-rs and hardened_malloc build under a Nix flake

[girlbossceo]

I don't know Nix but after attempting to use it with conduwuit via static feature, the build.rs file may require changes to support our "static builds" where we build the objects but use ar to create the static archive that gets linked later. I added the HOST_CC variable for the compiler, but ar is still missing so I feel like this is more complicated than it looks.

I also don't know if hardened_malloc itself has Nix support, but that might be the first step to this.