The logic to calculate the disk quota seems to be wrong

Summary

We got the following error in the overnight job:

Error instantiating artifact cache: Your system does not have enough available space to support the cache quota specified.

    You have specified a quota of infinity total disk space.
    The filesystem containing /root/.cache/buildstream/artifacts only has 618G total disk space.

Seems that either

• The logic to calculate free space is wrong
• The login to parse the quota options is wrong (It should not possible to request "infinite")

Steps to reproduce

simply check any of the overnight-tests jobs

Relevant logs and/or screenshots

https://gitlab.com/BuildStream/buildstream/-/jobs/159768027

Other relevant information

• BuildStream version affected: master