Initial stab at Buildkite CI config

[staticfloat]

The Buildkite pipeline is here: https://buildkite.com/julialang/packagecompiler-dot-jl

[DilumAluthge]

Ref #846

[staticfloat]

Next steps:

1. Use a rootfs image on Linux to provide compilers
2. Use a docker image on windows to provide compilers

[IanButterworth]

Next steps as in in a followup PR?

Also:

• Disable GHA CI

[sloede]

Right now, the Buildkite result details are not visible for unprivileged users (such as myself).

[staticfloat]

No, it has to be a part of this PR, as otherwise it's just constantly failing.

[DilumAluthge]

It looks like work on this PR has paused for now.

Currently, the packagecompiler-dot-jl Buildkite pipeline (https://buildkite.com/julialang/packagecompiler-dot-jl) is posting a failing (❌) commit status on every PR. This is expected, but it does add some visual noise when looking through the list of PRs.

@staticfloat @fredrikekre I am going to disable the Buildkite commit statusfor now. Once work on this PR has resumed, we can re-enable the Buildkite commit status.

To re-enable the Buildkite commit status, do the following steps:

1. Log in to Buildkite (log in to GitHub first, then go to https://buildkite.com/sso/julialang and click the green "Sign in to GitHub" button to SSO).
2. Go to the packagecompiler-dot-jl pipeline: https://buildkite.com/julialang/packagecompiler-dot-jl
3. Click on settings: https://buildkite.com/julialang/packagecompiler-dot-jl/settings
4. In the left-hand side, click on GitHub: https://buildkite.com/julialang/packagecompiler-dot-jl/settings/repository
5. Find the checkbox labeled "Update commit statuses", and check it.
6. Click the green "Save GitHub Settings" button at the bottom of the page.

(I can also perform those steps, if someone pings me.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.56%. Comparing base (b2e852b) to head (f68db05).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #849   +/-   ##
=======================================
  Coverage   84.56%   84.56%           
=======================================
  Files           3        3           
  Lines         823      823           
=======================================
  Hits          696      696           
  Misses        127      127           

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