or-tools: re-enable tests on x86_64-linux
This needs https://github.com/google/or-tools/pull/4745 to skip SCIP
tests if built without SCIP (which we currently do).
The tests currently still fail on aarch64-linux
(https://github.com/google/or-tools/issues/4746), so we only enable
for x86_64-linux. It can be re-tested with a more recent version once
or-tools is updated.