Error loading elements for freedesktop-sdk

Summary

Buildstream fails to load elements in the process of setting up the local cache quota. Affects buildstream master.

Steps to reproduce

• clone freedesktop: git clone https://gitlab.com/freedesktop-sdk/freedesktop-sdk and checkout 973d7bb340e605a2a143f2b1d3f2fdbf286dfd77
• apply https://gitlab.com/snippets/1893382
• Setup a bst-artifact-server
• bst build bootstrap/linux-headers.bst
• fails to load elements: https://gitlab.com/snippets/1893394

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