Set SGKIT_DISABLE_NUMBA_CACHE=1 by default

[jeromekelleher]

This is causing persistent problems, with cryptic segfaults being really really hard to debug with Dask (I just lost a whole morning to it!).

We have to turn off the numba cache until this is sorted out somehow.

[tomwhite]

Is changing the default in https://github.com/pystatgen/sgkit/blob/main/sgkit/accelerate.py#L6 sufficient?

[jeromekelleher]

I guess a warning if numba cache is enabled also? It's a horrible gotcha.

[jeromekelleher]

Partially addressed ion #1191.

To fully address I think we probably need to undocument this variable by removing the mention to it in the docs, and maybe add some comments in the source laying out the issue. Plus update to the changelog.