ci: drop custom NumPy builds for Python 3.15

[tirthpatel90]

Description

Fixes #2401

This PR removes custom NumPy build and wheel installation steps for Python 3.15 across CI workflow files (build-wheel.yml, test-wheel-linux.yml, and test-wheel-windows.yml) now that pre-built NumPy 3.15 nightly wheels are available.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[mdboom]

/ok to test b33bb7c

[github-actions]

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[mdboom]

It's going to be hard to evaluate this PR until the Python 3.15 is green on main again. Our self-built Numpy is segfaulting on import on main, and this upstream build of Numpy also is. It's great that the behavior matches, but we can't really evaluate that Numpy is working in CI until Numpy and/or CPython fixes the bug that is causing the segfault.

So we might just need to sit on this one for a while.

[seberg]

Thanks, but as the issue says, this should use the scientific-python-nightly-wheels not build from scratch.
Effectively, this just moved the build from build-time to test time. (Which pointed to it not being NumPy but rather an ABI change in CPython, but that is beside the point.)

Updating this will indeed not fix things since NumPy is also build with b2 but the main blocker for merging is that we should use the nightlies and not build from scratch.