tests: Wipe 'build/ARM' dir before running SystemC tests

It if the ARM code is changed, the building of the library for the
systemc tests can sometimes fail. This appears to have happened in
recent Nightly test failures: https://jenkins.gem5.org/job/nightly/237.

It's safer to just wipe the 'build/ARM' directory before running this
test.

Change-Id: I203c4e74c01215128b85bf69176921f8ecef4988
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60096
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>

tests/nightly.sh

@@ -179,6 +179,7 @@ cd -;
 build_and_run_SST RISCV opt
 
 build_and_run_systemc () {
+    rm -rf "${gem5_root}/build/ARM"
     docker run -u $UID:$GID --volume "${gem5_root}":"${gem5_root}" -w \
         "${gem5_root}" --memory="${docker_mem_limit}" --rm \
         gcr.io/gem5-test/ubuntu-20.04_all-dependencies:latest bash -c "\