fix syntax error in dockerfile

Author: pietroalbini

src/ci/docker/host-x86_64/test-various/Dockerfile

@@ -64,7 +64,7 @@ ENV NVPTX_TARGETS=nvptx64-nvidia-cuda \
     CC_nvptx64_nvidia_cuda=clang-11 \
     CXX_nvptx64_nvidia_cuda=clang++-11 \
     CFLAGS_nvptx64_nvidia_cuda=-flto \
-    CXXFLAGS_nvptx64_nvidia_cuda=-flto \
+    CXXFLAGS_nvptx64_nvidia_cuda=-flto
 ENV NVPTX_SCRIPT python3 /checkout/x.py --stage 2 test --host='' --target $NVPTX_TARGETS \
   tests/run-make \
   tests/assembly