Unhandled exception when trying to push to the bst CAS

Summary

When our CI tries to push to a public CAS server, runner both push/pull configurations, we are seeing and unhandled error exception.

 grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with (StatusCode.UNKNOWN, Exception calling application: [Errno 13] Permission denied: '/home/artifacts/artifacts/cas/tmp/tmps8kbywhm')>

Steps to reproduce

We have followed the test documentation for the CAS server and are using a digital ocean runner to build/push to the CI using bst version "dc9ec43b"

What is the current bug behavior?

Full log can be viewed here: https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/75575368

What is the expected correct behavior?

The artifact should be successfully pushed to the CAS

Relevant logs and/or screenshots

https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/75575368

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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