Missing tests for Linux-non-bwrap-non-FUSE situations

Summary

With !847 (merged) we no longer demand the presence of bubblewrap during installation on Linux. In addition, since !626 (merged) and !726 (merged) landed it makes sense for us to be able to run without it thanks to remote execution support. Unfortunately this means that we need to test what happens on Linux systems without bwrap/FUSE when attempting to perform local builds which need a sandbox to run.

Steps to reproduce

1. Have a Linux system with either too-old-bubblewrap (pre 0.1.2) or missing-bubblewrap or no /dev/fuse
2. Run a local build which uses the sandbox (i.e. not just stack/import/filter elements)
3. See the now handy error message explaining why the sandbox fails

What is needed to resolve this?

We need:

1. A test which runs on Linux in the absence of one or more of the above requirements
2. A docker container which is such a system
3. A change to .gitlab-ci.yml to use the above.

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.4