Revert differential-shellcheck.yml changes with resurrecting shellcheck vars

[nhosoi]

• Revert "replace action shellcheck.yml with differential_shellcheck.yml (#13)" This reverts commit 95ebac0a93f918f1a0be2107616ef69a766c780f. For more details, see https://github.com/linux-system-roles/network/pull/574.
• Add inventory/group_vars/shellcheck_roles.yml
• Resurrect the changes made in playbooks/update_files.yml from the previously reverted 95ebac0a93f918f1a0be2107616ef69a766c780f