Eliminate dependencies needed in `yaml:lint` CI job

What does this MR do?

Previously the job showed lots of unrelated warnings, such as those seen in https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/9520969036:

$ make lint-ci-yaml
env: can't execute 'bash': No such file or directory
/bin/sh: git: not found
/bin/sh: git: not found
/bin/sh: git: not found
make: go: No such file or directory

This was happening because make lint-ci-yaml loads the Makefile, which in turn executes bash scripts and other programs.

Simplify this by running prettier directly.