Configure the lint CI action to use tox

[phoevos]

Currently, our CI workflow manually configures a lint action to check the charm code:

katib-operators/.github/workflows/integrate.yaml

Lines 24 to 43 in a9c7609

	lint:
	name: Lint
	runs-on: ubuntu-20.04
	steps:
	- name: Check out code
	uses: actions/checkout@v3
	- name: Install dependencies
	run: |
	set -eux
	sudo apt update
	sudo apt install python3-setuptools
	sudo pip3 install black flake8
	- name: Check black
	run: black --check charms/*/src
	- name: Check flake8
	run: flake8 ./charms/*/src

This is unfortunate for a few reasons:

• It doesn't leverage the already configured tox test environments
• It uses different tools for linting (hence, testing locally with tox can produce different results)
• It leaves out a big part of the code, namely all tests

Goal

Configure the lint CI action to use tox, making sure that both the top-level (e.g. tests) and charm-specific (e.g. charms/katib-controller/tests) directories are checked.