CMake: drop invalid OPTIONAL from Cycles runtime-deps install

install(DIRECTORY) disallows OPTIONAL together with FILES_MATCHING, which
broke reconfigure. The directory is always present here (produced by the
CyclesBuild step), so OPTIONAL was unnecessary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CMakeLists.txt

@@ -402,11 +402,12 @@ if(OPENUSD_HAS_CYCLES)
         DESTINATION bin/usd
         OPTIONAL
     )
-    # Cycles runtime deps (embree4, epoxy, imath, openvdb, OIDN, sycl, etc.)
+    # Cycles runtime deps (embree4, epoxy, imath, openvdb, OIDN, sycl, etc.).
+    # No OPTIONAL: install(DIRECTORY) disallows it with FILES_MATCHING, and the
+    # dir always exists here since it is produced by the CyclesBuild step.
     install(DIRECTORY "${CYCLES_INSTALL_DIR}/"
         DESTINATION bin
         FILES_MATCHING PATTERN "*.dll"
-        OPTIONAL
     )
     # tbb12.dll (used by openvdb/embree4) lives in the precompiled lib tree, not install output
     install(FILES "${CYCLES_TBB12_DLL}"