build: exclude markdown files from some GitHub Actions

Ignore all markdown files when determining if test-asan, coverage-*, and
test-macos need to run.

Refs: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#patterns-to-match-file-paths

PR-URL: https://github.com/nodejs/node/pull/39565


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>