chore(tvix/eval): add rustfmt to the shell

Change-Id: Ia049bf08feeb1a75f30df8fed7f88fcdf650521b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6663
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/shell.nix

@@ -7,6 +7,7 @@ pkgs.mkShell {
   packages = [
     pkgs.cargo
     pkgs.rustc
+    pkgs.rustfmt
     pkgs.clippy
   ];
 }