Replace fixture generation through exec() with a cleaner method

[gbionescu]

Currently, image fixtures are built by enumerating capabilities in the S3 bucket and calling exec() to create new fixtures.

It would be much cleaner to have a list of images inside the test framework that generates the fixtures for us.

Documentation will also have to be updated: https://github.com/firecracker-microvm/firecracker/blob/41c9cdb00d1061b6f7a67b0cf754edd2ebdd069c/tests/README.md#adding-microvm-images

[kumargu]

Happy to help on the issue.

[roypat]

This will be fixed by #3896

[roypat]

Above PR has been merged. With the new CI artifacts, we no longer generate fixtures through exec