Add buildkite configuration

[devmotion]

Adds an initial buildkite configuration to fix CI errors.

Basically the same as https://github.com/JuliaGaussianProcesses/AbstractGPs.jl/pull/335.

[devmotion]

If you have 2 mins to make proper use of the GROUP variable in runtests also, so that buildkite isn't running the tests, that would be ideal.

Ah, I assumed that we already use GROUP. I'll fix it.

[codecov[bot]]

Codecov Report

Base: 93.33% // Head: 0.00% // Decreases project coverage by -93.33% :warning:

Coverage data is based on head (4f00074) compared to base (23ceaf7). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #138 +/- ## ========================================== - Coverage 93.33% 0.00% -93.34% ========================================== Files 4 4 Lines 285 280 -5 ========================================== - Hits 266 0 -266 - Misses 19 280 +261 ``` | [Impacted Files](https://codecov.io/gh/JuliaGaussianProcesses/ApproximateGPs.jl/pull/138?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses) | Coverage Δ | | |---|---|---| | [src/LaplaceApproximationModule.jl](https://codecov.io/gh/JuliaGaussianProcesses/ApproximateGPs.jl/pull/138/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses#diff-c3JjL0xhcGxhY2VBcHByb3hpbWF0aW9uTW9kdWxlLmps) | `0.00% <0.00%> (-96.90%)` | :arrow_down: | | [src/TestUtils.jl](https://codecov.io/gh/JuliaGaussianProcesses/ApproximateGPs.jl/pull/138/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses#diff-c3JjL1Rlc3RVdGlscy5qbA==) | `0.00% <0.00%> (-94.12%)` | :arrow_down: | | [src/SparseVariationalApproximationModule.jl](https://codecov.io/gh/JuliaGaussianProcesses/ApproximateGPs.jl/pull/138/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses#diff-c3JjL1NwYXJzZVZhcmlhdGlvbmFsQXBwcm94aW1hdGlvbk1vZHVsZS5qbA==) | `0.00% <0.00%> (-90.25%)` | :arrow_down: | | [src/utils.jl](https://codecov.io/gh/JuliaGaussianProcesses/ApproximateGPs.jl/pull/138/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses#diff-c3JjL3V0aWxzLmps) | `0.00% <0.00%> (-50.00%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGaussianProcesses)

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[willtebbutt]

@devmotion thanks for sorting this. Please feel free to merge once CI passes