skip unused and duplicate workflow runs

[gpelouze]

Using https://github.com/marketplace/actions/skip-duplicate-actions, we skip actions:

• skip everything if a successful identical workflow run exists
• skip everything if only a README.md is modified
• skip flavor if only files from another flavor were modified (only builds my-flavor if files are modified in .github/, docker/, or flavors/my-flavor/)
• do not skip anything on release, workflow_dispatch and schedule