Make sure TARGET_OS="linux" is set for validate_docker_images.yml

Author: atalman

.github/workflows/validate_docker_images.yml

@@ -36,4 +36,5 @@ jobs:
         set -ex
         export MATRIX_GPU_ARCH_TYPE="cuda"
         export MATRIX_GPU_ARCH_VERSION="${{ matrix.cuda }}"
+        export TARGET_OS="linux"
         python test/smoke_test/smoke_test.py --package torchonly --runtime-error-check enabled