Fix CI failure

Tristan Maatrequested to merge
fix-random-ci-failure into master

This should fix the slew of CI issues buildstream has been having, e.g. https://gitlab.com/BuildStream/buildstream/-/jobs/58179654.

We still aren't quite sure what caused setuptools_scm to be missing, but presumably our --index-url hack isn't quite working as we expect it to.

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