Local cache size calculation is wrong

Summary

Trying to run bst build fails because my local cache cannot be instanciated because of a lack of space.

Steps to reproduce

• Give a cache size bigger than what you are using and bigger than your total file system free space
• run a bst build command

What is the current bug behavior?

Error instantiating artifact cache: Your system does not have enough available space to support the cache quota specified. 
You have specified a quota of 95% total disk space.
The filesystem containing /home/bschubert/.cache/buildstream/artifacts only has: 39.3% of total disk space available.

What is the expected correct behavior?

The cache should be able to be instanciated

Relevant logs and/or screenshots

Information about the system:

> mount | grep .cache
/dev/vdb on /home/bschubert/.cache

> ls ~/.cache
buildstream

> lsblk
vdb 1000G /home/bschubert/.cache

> du -hs /home/bschubert/.cache/buildstream/*
547G artifacts
4.0K build
440M logs
# I had manually removed the sources first

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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