Add in-tree buildkite pipeline

[mvines]

No description provided.

[mvines]

@garious or @aeyakovenko - anything that looks alarming here? I'm trying to merge the current state of the various Travis and Buildkite pipelines into one with this PR and I think it works (as well as the current system does).

Might be helpful to look at the Environment Variables at https://buildkite.com/solana-labs/solana/settings for context too.

The status checks for this PR are a mess. Once this PR lands most of them will be removed and we'll only see:

• ci-gate nag (helpful for folks without write access so they know they need help to get their PR into buildkite CI)
• buildkite/solana - only accessible to folks with a buildkite account
• PUBLIC LOG: buildkite/solana - public version of ^, visible to the world

[garious]

Looks good. You got kcov to work without docker?

[mvines]

You got kcov to work without docker?

It seems to get much further on Buildkite than it did on Travis at least (probably due to existing software already installed on the sagan build agent, which is a larger issue we'll need to deal with when it's time to scale CI to multiple machines).

The codecov.io setup seems to be busted though, here's the cov report from this PR and it's pretty angry: https://codecov.io/github/aeyakovenko/solana/commit/34514d65bc46028e6b3f8e66181033f49e10bd36.
Seems like this hasn't been working for quite a while, I doubt this is relate to this PR. Assuming #177 turns out, do we want codecov.io at all?