CI: Cache artifacts generated on the `main` & `11.8.x` branches

[leofang]

Instead of using the upload-artifact action, we should use save-cache action so that the main-built artifacts can be referenced and reused by later workflows.

[leofang]

This blocks, e.g. #294 and #299.

[leofang]

This also blocks #356.

[leofang]

Update: see the new approach explained here without needing an actual global storage/cache.