Switch to workflow calls

[MOZGIII]

This PR changes the CI structure.

Pros:

• more structured and idiomatic according to Github Actions
• less code repetition, DRY
• better extensibility and flexibility

Cons:

• the bug with / in the action names at the UI (we have worked around it with @ symbol and reverse order of the mode / platform)
• work duplication between potentially reused runs (i.e. release and build docker jobs) - yes, but really we'd want them to be retriggered either way because we want them to run with separate refs - and then it's all about the caches to eliminate extra work