CI: Fix Meson mixup between Conda and system Python headers #2687

Job	Run time
Build source distribution	58s
Build wheel on ubuntu-latest	3m 1s
Build wheel on ubuntu-22.04-arm	2m 38s
Build wheel on windows-latest	4m 15s
Build wheel on macos-15-large	2m 6s
Build wheel on macos-14	1m 18s
test_dist_pypi	8s
upload_pypi	-1s
upload_nightly	-1s
	14m 22s

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