`base/base-alpine.bst: Directory not found`

Is this a known issue? This only seems to permanently affect test-fixed-debian-9 although I've seen other temporary failures on other jobs (with different images), generally in test_deterministic_source_umask[tar]: https://gitlab.com/BuildStream/bst-plugins-experimental/-/jobs/506395885#L714

[--:--:--] START   base/install-dpkg.bst: Caching artifact
[00:00:11] FAILURE base/install-dpkg.bst: Caching artifact
[00:00:33] FAILURE base/install-dpkg.bst: Directory not found in local cache: [Errno 2] No such file or directory: '/builds/BuildStream/bst-plugins-experimental/cache/integration-cache/cache-zlwnfebk/cas/objects/b7/a215dd58ad079356f2d362f6561fb6dbcffa2467d099b6a0d4cd04e3a98aea'

this is blocking !91 (merged)