.github: fallback on rebuild-testbench -j 1 when failing

Parallel builds are fast but they bury errors, for instance good luck
finding the cause of this failure:
 https://github.com/thesofproject/sof/actions/runs/7667629748/job/20897861756?pr=8019

Use the well known technique of falling back on a single thread when
failing.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

.github/workflows/testbench.yml

@@ -49,7 +49,8 @@ jobs:
              ./scripts/build-tools.sh -t
 
       - name: build testbench
-        run: ./scripts/rebuild-testbench.sh
+        run: ./scripts/rebuild-testbench.sh ||
+             ./scripts/rebuild-testbench.sh -j 1
 
       - name: run testbench
         run: ./scripts/host-testbench.sh