Option to add filter list for buildifier workflow

[c-parsons]

For the bazelbuild/skydoc repository, I'd like to introduce a number of Starlark "testdata" bzl files. These files are for testing purposes only, and, by design, they will not always abide by buildifier's suggestions. (for example, some files will have no top-level docstring.) It would be great if I could disable the buildifier CI workflow for any files under this testdata/ directory.