ci: build PhysX SDK 5.6.1 (was 5.6.0)

Bump the checked-out NVIDIA-Omniverse/PhysX source ref from 107.0-physx-5.6.0
to 107.3-physx-5.6.1. 5.6.1 fixes "a crash when a pair of actors had lots of
colliding shapes" (matches the gripper-fingers + cube sustained-grasp crash in
the RL lift task) and adds PxSceneFlag::eSOLVE_ARTICULATION_CONTACT_LAST.

CUDA arch config unchanged (sm_75 + PTX JIT to newer arches) so the 5.6.0/5.6.1
A/B isolates the SDK version, not the arch target.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Lumengine

.github/workflows/BuildPhysX.yml

@@ -34,7 +34,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: NVIDIA-Omniverse/PhysX
-          ref: 107.0-physx-5.6.0
+          ref: 107.3-physx-5.6.1
 
       # === WINDOWS ===
       - name: Install CUDA Toolkit (Windows)