Turn off numba cache by default

[jeromekelleher]

Partial fix for #1156

This turns off numba caching by default, and at least lets us use the current git version of sgkit without hitting this issue repeatedly.

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.

[jeromekelleher]

Hmm, it seems like there's an unfortunate side effect here of turning off caching. With caching turned off, when we run the functions with NUMBA_DISABLE_JIT=1 to get coverage we get a whole bunch of numba errors and deprecations that don't happen when caching is on.

I'm not sure what to do here, there's quite a few deprecations and errors that seem non-trival to fix. For example, we seem to hit this problem https://github.com/numba/numba/issues/8886 which is non-trival to workaround.

Any thoughts @tomwhite, @timothymillar?

[jeromekelleher]

I worked around this here by explicitly turning on caching, so that we can get our builds to pass and at least have our code not segfault by default. Opened #1194 to track the CI issue.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a386541) 100.00% compared to head (5c85a92) 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1191 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 52 52 Lines 5210 5210 ========================================= Hits 5210 5210 ```

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