Fix tests/artifactcache/expiry.py::test_expiry_order on bst-1.2

Summary

Test has been disabled by !1436 (merged). We need to investigate why it fails on the builders but not on developer machines. Eventually if there is a good reason, try to find a way to work around it by allowing it to fail only the builders.

Steps to reproduce

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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