tests,systemc: Fix nightly systemc test

This fixes these nightly failing tests:
https://jenkins.gem5.org/job/nightly/609/

Due to this commit:
https://gem5-review.googlesource.com/c/public/gem5/+/68758
The source files are not copied to the "build" directory by default.
This caused the systemc tests to fail as the
"util/systemc/gem5_within_systemc/Makefile" depends on generated source
files in the "build" directory.

This patch adds the "--duplicate-sources" flag to the building of the
ARM binaries necessisary for running systemc. The README has been
updated to reflect this.

Change-Id: I3006005e43276097be98f7d4685f3d98c180d3f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70860
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

tests/nightly.sh

@@ -173,9 +173,11 @@ build_and_run_systemc () {
     docker run -u $UID:$GID --volume "${gem5_root}":"${gem5_root}" -w \
         "${gem5_root}" --memory="${docker_mem_limit}" --rm \
         gcr.io/gem5-test/ubuntu-22.04_min-dependencies:${tag} bash -c "\
-scons -j${compile_threads} --ignore-style build/ARM/gem5.opt && \
-scons --with-cxx-config --without-python --without-tcmalloc USE_SYSTEMC=0 \
-    -j${compile_threads} build/ARM/libgem5_opt.so \
+scons -j${compile_threads} --ignore-style --duplicate-sources \
+build/ARM/gem5.opt && \
+scons --with-cxx-config --without-python --without-tcmalloc \
+--duplicate-sources USE_SYSTEMC=0  \
+-j${compile_threads} build/ARM/libgem5_opt.so \
 "
 
     docker run -u $UID:$GID --volume "${gem5_root}":"${gem5_root}" -w \