Race condition when calculating disk usage

Summary

There's an apparent race condition when calculating disk usage on a directory. See the log.

Steps to reproduce

1. Clone freekdesktop-sdk.
2. Remove artifact servers configuration from bootstrap/project.conf and sdk/project.conf.
3. Run make

Note: I only reproduced this bug when running the above steps in the CI. It build successfully on my local machine in the first attempt.

What is the current bug behavior?

Relevant logs and/or screenshots

https://gitlab.com/BuildStream/buildstream/-/jobs/84724566

• BuildStream version affected: /milestone %BuildStream_v1.x

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