pull out common pieces of yaml in the buildkite pipeline

[toshok]

This was initially motivated by what I saw as a bunch of needless repetition in plugins due to the buildevents work in #943, but I found other things that could be pulled out as well.

Found a pretty big 🤦 in that yaml can't seem to deal with anchors to and merging references of lists. This causes a bunch of pain in both the commands and plugins anchors that I'll call out inline, and it means we can't express plugins/commands in the steps using the syntax that agents/env uses.

[toshok]

Ideally I would be able to express this as:

- &buildevents_plugins
  - seek-oss/aws-sm#v2.3.1:
      region: us-east-2
      env:
        BUILDEVENT_APIKEY: honeycomb-api-key
        BUILDEVENT_BUILDKITE_API_TOKEN: buildkite-api-token-honeycomb-build-events
  - replayio/buildevents#adb8a05: ~

and then reference the anchored list below like:

  - &linux_plugins
    plugins:
      - thedyrt/skip-checkout#v0.1.1:
          cd: /home/ubuntu/chromium/src
      <<: *buildevents_plugins

but alas, that isn't valid yaml. instead each plugin needs its own anchor and those get mentioned individually in the os plugins lists.

[toshok]

q: is there a reason not to use buckle everywhere? that would make it so we can reuse the same build command sequence across linux/macos.

[toshok]

the buck2/buckle build anchors are identical except for two lines. in fact except for two words. This is another place where yaml list merging (and string concatenation) would help a bunch. I didn't really want to create anchors for each shared command here, like I did up in plugins.

[jazzdan]

Eventually we'll be able to use buckle everywhere, but there's currently a bug in the Windows version benbrittain/buckle#21

[toshok]

the diff here is pretty hard to read. probably easier to look at the resulting file.

[jazzdan]

LGTM!

[toshok]

rebased off master ☝️

[toshok]

and one last rebase to make sure nothing's worse off than before, before I land.