fix: load metadata from charmcraft.yaml in profile integration tests

[tonyandrewmeyer]

Since charmcraft init doesn't create a metadata.yaml file any more, the integration tests need to load the metadata from the charmcraft.yaml file, since they typically run from the source folder rather than within a deployed environment.

[lengau]

Hi, and thanks for this!

This is the same as #1468 so I'm going to close this in favour of it