_sandboxreapi.py: Support read-only root

Jürg Billeterrequested to merge
juerg/reapi-readonly into master

This implements support for read-only root in SandboxREAPI for buildbox-run and remote execution. This depends on BuildGrid/buildbox/buildbox-fuse!31 (merged) and BuildGrid/buildbox/buildbox-casd!130 (merged).

This will allow tests/integration/script.py::test_script_no_root to pass with buildbox-run-bubblewrap. However, I'm deferring the drop of the xfail until the next update of buildbox-fuse in the docker images used in CI.

This is part of #719 (closed).

Edited by Jürg Billeter

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