Fix coverage workflow

[reuvenperetz]

Pull Request Description:

Use separate environments for coverage testing of Keras and Torch.
This fixes the issue of the flag "FOUND_TF" being set even if TF is uninstalled. The issue started with the start of pytest usage.

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).

[ofirgo]

Just to make sure - this produces a single coverage report, based on both test frameworks run?