yamllint: Run yaml linter only on modified files in pre-commit.

With the parameter `args: ['.']`, yamllint would run over every
file during pre-commit, including those not being commited, and it
would allow for false negatives, not allowing a commit, even if
commited yaml files had no issues, but another file, not par of the
commit, had.

By changing the yamllint parameter to `files: \.(yaml|yml)$` it
will only check files being commited, preventing false negatives,
and allowing for faster commits.