Build(Cargo): Add mold as default linker in both profiles

I measured the linking times by running `cargo rustc --bin trinitrix --
-Z time-passes`, which resulted in following data:

`ld`:
37.794s to link
`mold`:
21.389s to link

The downside for now is, that this requires to have mold installed to
build trinitrix (The nix flake based direnv already has it) and that
mold only works on linux.

.cargo/config.toml

@@ -0,0 +1,2 @@
+[target.x86_64-unknown-linux-gnu]
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]

flake.nix

@@ -52,6 +52,7 @@
 
       nativeBuildInputs = with pkgs; [
         pkg-config
+        mold
       ];
       buildInputs = with pkgs; [
         openssl