lava: Switch to `os: apertis` after upgrade to LAVA 2021.05

With the update to LAVA 2021.05 a new deployment type has been added to ensure that things work despite:

1. /bin/sh not pointing to Bash
2. a read-only / that prevents unpacking the overlay there

This does not currently seem to cause issue with the tests in this repository, but it's nice to do things in the same way as the main test harness to avoid confusion later on, see tests/apertis-test-cases@349e2525

See https://gitlab.collabora.com/lava/lava/-/commit/d621f93191591714449e67945ebd95cb6396727a for the commit introducting the apertis type.