fix: add workspace.metadata.crane.name to silence crane warnings

When users import the ngit-grasp flake into their NixOS configurations,
crane emits warnings about not being able to find the package name in
the workspace Cargo.toml. This adds the recommended workspace.metadata
section to explicitly specify the crane package name for the workspace.

Fixes the warning:
  evaluation warning: crane will use a placeholder value since name
  cannot be found in /nix/store/.../Cargo.toml

This is a cosmetic fix that doesn't affect functionality but improves
the user experience when importing the flake.

Cargo.toml

@@ -79,3 +79,6 @@ path = "src/main.rs"
 
 [workspace]
 members = [".", "grasp-audit"]
+
+[workspace.metadata.crane]
+name = "ngit-grasp"