CI/TST: fix tests for compat with numpy 2+ when using legacy promotion
This is trying to fix the current failures on main.

It seems that our test runs were still using numpy 1.26 (because of some downgrade after initial env creation because of removing nomkl), but that stopped being the case some hours ago. This turns up a bunch of failures in our tests when running hem with numpy >= 2 ánd with `NPY_PROMOTION_STATE=legacy`. 

We should properly fix this situation (stop using the legacy promotion state), but this PR should already get CI green again.
This does not yet fix the macos and arm failure (those are also failing on main, and separate issues) > This does not yet fix the macos and arm failure (those are also failing on main, and separate issues)

-> https://github.com/pandas-dev/pandas/pull/60143 > We should properly fix this situation (stop using the legacy promotion state), but this PR should already get CI green again.

Saw this randomly.  Is that hard?  I mean, I think you can just remove any setting of `NPY_PROMOTION_STATE` from CI, and if you still have issues, then you don't have to worry one way or another.

(I was planning to remove support for it in 2.2, I thought I already did in the nightlies, but maybe not.) No idea, let's see in https://github.com/pandas-dev/pandas/pull/60144

(I was planning to open an issue about it, because in addition to CI I think there is also the discussion about "following" the new behaviour of numpy) Well, FYI, I was planning to remove this in NumPy 2.2.  I would have guessed that I already did in the nightlies...
This is a debugging/transition feature, I suppose it is nice if you support it to some degree (since someone who uses it for testing may also use pandas), but overall... > Well, FYI, I was planning to remove this in NumPy 2.2. I would have guessed that I already did in the nightlies...

Yeah, we already ran into `np._get_promotion_state` being removed, but that is fixed (https://github.com/pandas-dev/pandas/pull/59818). 
AFAIU the main reason we still have this env variable set is for numpy 1.26 In any case, it seems to be working fine to completely remove the usage of the env variable -> https://github.com/pandas-dev/pandas/pull/60144 Closing this in favor of https://github.com/pandas-dev/pandas/pull/60144