CI: Use wheel to install base requirements

[cdce8p]

Proposed change

Noticed that the prepare-base job wasn't using wheel to build build packages.

Using legacy 'setup.py install' for python-slugify, since package 'wheel' is not installed.
...

We already install wheel for the prepare-tests job, so it does make sense to do the same for prepare-base.

core/.github/workflows/ci.yaml

Lines 548 to 551 in 6dcec89

	pip install -U "pip<20.3" setuptools wheel
	pip install -r requirements_all.txt
	pip install -r requirements_test.txt
	pip install -e .

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