chore: add gnu as source for cl-lib

[vidbina]

Adding gnu as a source to resolve the "Some dependencies were not available:
cl-lib" issue.

Debugger entered--Lisp error: (error "Some dependencies were not available: cl-lib")
  error("Some dependencies were not available: %s" "cl-lib")
  (let ((missing-dependencies (cdr err))) (error "Some dependencies were not available: %s" (s-join ", " (mapcar #'symbol-name (mapcar #'cask-dependency-name missing-dependencies)))))
  (condition-case err (progn (cask-install (cask-cli--bundle))) (cask-missing-dependencies (let ((missing-dependencies (cdr err))) (error "Some dependencies were not available: %s" (s-join ", " (mapcar #'symbol-name (mapcar #'cask-dependency-name missing-dependencies)))))) (cask-failed-initialization (let* ((data (cdr err)) (message (error-message-string (nth 0 data))) (output (nth 1 data))) (error "Package initialization failed: %s\nOutput:\n%s" message output))) (cask-failed-installation (let* ((data (cdr err)) (dependency (progn (or (progn ...) (signal ... ...)) (aref (nth 0 data) 1))) (message (error-message-string (nth 1 data))) (output (nth 2 data))) (if dependency (error "Dependency %s failed to install: %s\nOutput:\n%s" dependency message output) (error "Package installation failed: %s\nOutput:\n%s" message output)))))
  cask-cli/install()
  commander--handle-command(("install"))
  commander-parse(("install"))
  (if commander-parsing-done nil (commander-parse (or commander-args (cdr command-line-args-left))))
  load-with-code-conversion("/nix/store/1cnsrsjwz325gmkgyb5snvf7ph1axzhk-cask-0..." "/nix/store/1cnsrsjwz325gmkgyb5snvf7ph1axzhk-cask-0..." nil t)
  command-line-1(("-scriptload" "/nix/store/1cnsrsjwz325gmkgyb5snvf7ph1axzhk-cask-0..." "--" "install"))
  command-line()
  normal-top-level()