Arm backend: Remove running examples/arm/setup.sh in the github docker

This will move Arm PR testing to use an empty docker and
examples/arm/setup.sh will always run from a fresh setup when testing a
PR to minimize the risk of and old run of examples/arm/setup.sh done during docker creation colliding PR testing, or being out of sync with it.
The different runs of examples/arm/setup.sh also used different
install folders and that might break stuff even more.

Signed-off-by: Zingo Andersen <zingo.andersen@arm.com>
Change-Id: I795078d3d764a3f7ad5d6043fb8b70290af5789f