CI: use conda to create Python environments instead of GitHub Actions Python matrix

[tswast]

By using conda, we should be able to use an editable install without all the permissions issues we were encountering in #38 This should allow us to remove this hack:

ibis-bigquery/ci/install_deps.sh

Lines 6 to 10 in c3d188e

	# See https://github.com/pypa/pip/issues/7953
	echo "import site
	import sys
	site.ENABLE_USER_SITE = '--user' in sys.argv[1:]
	$(cat ./ibis/setup.py)" > ./ibis/setup.py

See: ibis-project/ibis#2738