maint: put cargo completions in the dev shell

package.nix

@@ -127,9 +127,14 @@ in {
     mkShell,
     python3Packages,
     fenixToolchain,
+    cargo,
   }: let
     mkShell' = mkShell.override { inherit stdenv; };
     pyEnv = python3Packages.python.withPackages (p: [ p.beartype ]);
+    # Fenix's Cargo doesn't have completions, but Nixpkgs' does.
+    cargoCompletions = linkFarm "cargo-bash-completions" {
+      "share/bash-completion" = cargo + "/share/bash-completion";
+    };
   in mkShell' {
     name = "devshell-for-${self.name}";
     inputsFrom = [ self ];
@@ -137,12 +142,15 @@ in {
       pyEnv
       stdenv.cc
       fenixToolchain
+      cargoCompletions
     ];
     env.PYTHONPATH = [
       "${pyEnv}/${pyEnv.sitePackages}"
       # Cursed.
       "${path}/nixos/lib/test-driver/src"
     ] |> lib.concatStringsSep ":";
+
+    passthru = { inherit cargoCompletions; };
   };
 
   passthru.modulesPath = self.modules + "/share/nixos/modules";