install diff-filter-buildkite-plugin to only build when relevant files changed

[fatteneder]

This adds a filter that skips CI if a PR only changed things like README.md, HISTROY.md etc.

This is a first attempt towards fixing #243.
Still lacks a few desired features like (list taken from the recent Slack thread, @DilumAluthge)

• manually ignoring the filter for pre-releases and tags,
• ignore the filter when merging into master.

[IanButterworth]

I believe NEWS.md and HISTORY.md are in the docs, so I think it needs at least the docs to be built for those?

[IanButterworth]

Does this mean the docs run is only run for those matches? Don't we need to cover changes to docstrings and their doctests?

[DilumAluthge]

Yeah. And docstrings can be in any file. I think we pretty much always need to build the docs.

[fatteneder]

Docs build should be triggered by any of these two now:

 if [[ $${BUILDKITE_PLUGIN_DIFF_FILTER_TRIGGERED_BUILD} == 1 ||
                $${BUILDKITE_PLUGIN_DIFF_FILTER_TRIGGERED_DOC} == 1   ]]; then

But on that note: Do we always need at least one Julia build to also be able to build the docs? That's how I understand the instructions here #243 (comment). If so, then that's not yet handled here.

---

Perhaps I should revert the diff-filter-doc for the moment, then just not ignore HISTROY.md and NEWS.md for diff-filter-build, and always build the docs when we build julia. A follow-up PR could then experiment with refinements.

[DilumAluthge]

Yeah, we need at least one build of Julia in order to do the docs job.

I would say that it's okay to just do that one build (as well as the docs build) unconditionally, regardless of the diff.