ci: fix line breaks in check scripts and sanitize commit hash

- Updated .gitlab/templates/linting.yml to use YAML block scalars (|-)
  for all check scripts to avoid introducing leading spaces on line breaks.
- Added commit_hash.strip() in check_style.py and check_copyright.py
  to prevent accidental whitespace in Git arguments.

Signed-off-by: Wei-Chia Su <Wei-Chia.Su@arm.com>