set locale in CI

[beckermr]

It appears that we have matrix jobs on two locales, but as far as I can tell, we don't ever set those locales in the CI jobs.

[cvanelteren]

The twin is a bit overkill but we test locale settings, see https://github.com/Ultraplot/ultraplot/blob/258561e957d41dfb35c6922578cc8e71f1cf75a2/ultraplot/tests/test_format.py#L184

[beckermr]

OK So we remove locale from the build matrix?

[cvanelteren]

It need one, otherwise that test would fail. The other can be removed. I would say keep the utf8