Do not install test dependencies in Install step

[radarhere]

#9945 updated the Makefile so that the Build step will install the test dependencies.

Pillow/.ci/build.sh

Line 6 in afb1633

make install-coverage

Pillow/Makefile

Lines 64 to 65 in afb1633

	install-coverage:
	CFLAGS="-coverage -Werror=implicit-function-declaration" python3 -m pip -v install .[tests]

Pillow/pyproject.toml

Lines 65 to 80 in afb1633

	optional-dependencies.tests = [
	"coverage>=7.4.2",
	"defusedxml",
	"markdown2",
	"olefile",
	"packaging",
	# Only used by the leak tests, which only run on Linux and macOS.
	# psutil does not support iOS at all, and ships no wheels for Android.
	"psutil; sys_platform=='linux' or sys_platform=='darwin'",
	"pytest",
	"pytest-cov",
	"pytest-timeout",
	"pytest-xdist",
	"setuptools",
	"trove-classifiers>=2024.10.12",
	]

This means that the Install step doesn't need to install those dependencies beforehand.