flatpak-autotools example test doesn't run

Summary

Now that the ostree repo is outside of the main repo, the flatpak-autotools test always skips as the ostree plugin isn't installed.

Steps to reproduce

run: tox -- --integration tests/examples/flatpak-autotools

Possible fixes

I think we want to try and install the ostree plugin to tox in order to run this test.

Other relevant information

• BuildStream version affected: master

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