CI: Cirrus CI - skip build on ignore-paths

Cirrus CI supports a `<job>.skip` key that skips the build if the expression
evaluates to true. This adds the same list of on.pull_request.paths-ignore
patterns from GitHub Actions workflows.

This should save several minutes of CI times on PRs when the changes are
only in the README/doc files.

Author: Ayesh

.cirrus.yml

@@ -5,6 +5,7 @@ freebsd_task:
   name: FREEBSD_DEBUG_NTS
   freebsd_instance:
     image_family: freebsd-13-2
+  skip: "changesIncludeOnly('docs/**', 'NEWS', 'UPGRADING', 'UPGRADING.INTERNALS', '**/README.*', 'CONTRIBUTING.md', 'CODING_STANDARDS.md', '.travis.yml', 'travis/**', '.circleci/**')"
   env:
     ARCH: amd64
   install_script: