Cache .tox environment
In cki-tools, the unit tests running via tox install quite a lot of dependencies, which takes it sweet time (nearly 7 minutes total runtime).
To speed that up, investigate caching the .tox environment.
Not in scope is extending that for other repos. Using it in cki-tools for a while should provide the experience needed to make it robust enough to extend it to other repos via the gitlab templates in cki-lib.
AC:
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enable cache for tox in MRs: !948 (merged)