feat(tvix): add crate2nixGenerate target

`mg run //tvix:crate2nixGenerate --` runs crate2nix generate and
depotfmt's it afterwards.

This removes a frequent point of friction, because the Cargo.nix emitted
by crate2nix needs another formatter.

Change-Id: I649495980718cd3847d4cff77c9d4bfcb599387c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7612
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/default.nix

@@ -1,5 +1,5 @@
 # Nix helpers for projects under //tvix
-{ pkgs, ... }:
+{ pkgs, depot, ... }:
 
 {
   # Load the crate2nix crate tree.
@@ -8,6 +8,13 @@
     nixpkgs = pkgs.path;
   };
 
+  # Run crate2nix generate in the current working directory, then
+  # format the generated file with depotfmt.
+  crate2nixGenerate = pkgs.writeShellScriptBin "crate2nix-generate" ''
+    ${pkgs.crate2nix}/bin/crate2nix generate
+    ${depot.tools.depotfmt}/bin/depotfmt Cargo.nix
+  '';
+
   # Provide a shell for the combined dependencies of all Tvix Rust
   # projects. Note that as this is manually maintained it may be
   # lacking something, but it is required for some people's workflows.