CI: fix version of audformat by fetching tags

[hagenw]

At the moment, the CI jobs are just checking out the last commit of the repository. As we use git tags to get the version of audformat, this means the version of audformat after running pip install -r requirements.txt starts always with 0.1-dev. Which means when running pip install -r tests/requirements.txt afterwards another version of audformat is installed as we install audb which requires >=1.2.0.

Now it installs

and does not need to install another version of audformat when running pip install -r tests/requirements.txt.