[CI] CBoostedTreeTest.testLinear fails consistently in linux-aarch64 debug builds

This test has failed 4 days in a row now:

1. https://elasticsearch-ci.elastic.co/job/elastic+machine-learning+master+linux-aarch64+debug/12/
2. https://elasticsearch-ci.elastic.co/job/elastic+machine-learning+master+linux-aarch64+debug/13/
3. https://elasticsearch-ci.elastic.co/job/elastic+machine-learning+master+linux-aarch64+debug/14/
4. https://elasticsearch-ci.elastic.co/job/elastic+machine-learning+master+linux-aarch64+debug/15/

The error is:

```
[Exception] - difference{1.90174} between 0.0{0} and modelBias[i][0]{-1.9017438598704961} exceeds 1.78885
== [File] - CBoostedTreeTest.cc
== [Line] -549
```

The test doesn't fail like this with compiler optimisation enabled.

This needs investigating as it may be a sign of undefined behaviour that could bite us in some way that's not covered in tests.