shell.nix: include gh in the default shell

GitHub's CLI is now used by nixpkgs-review to fetch Nixpkgs evaluation
results from GitHub Actions. Including it in the default shell will
allow all contributors to easily log-in to GitHub when nixpkgs-review
tries to fetch evaluation results.

Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>

shell.nix

@@ -35,5 +35,8 @@ curPkgs
     # Helper to review Nixpkgs PRs
     # See CONTRIBUTING.md
     nixpkgs-review
+    # Command-line utility for working with GitHub
+    # Used by nixpkgs-review to fetch eval results
+    gh
   ];
 }