CI: Fix Meson mixup between Conda and system Python headers #2692

Job	Run time
Build source distribution	1m 3s
Build wheel on macos-15-large	2m 4s
Build wheel on ubuntu-22.04-arm	2m 44s
Build wheel on windows-latest	4m 55s
Build wheel on macos-14	1m 56s
Build wheel on ubuntu-latest	3m 8s
test_dist_pypi	7s
upload_pypi	0s
upload_nightly	0s
	15m 57s

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