Add target os check if TARGET_OS is not set (#1002)

Summary:
Fixes crash when TARGET_OS is not set

Pull Request resolved: #1002

Reviewed By: NivekT, ejguan

Differential Revision: D43164646

Pulled By: atalman

fbshipit-source-id: 5ac4ef1a5bf4250db1314e9cca3250a75687450b

Author: atalman

.github/scripts/validate_binaries.sh

@@ -8,7 +8,7 @@
 AWS_ENABLED=1
 if [[ ${MATRIX_PACKAGE_TYPE} = "conda" ]]; then
     conda install -y torchdata -c "${PYTORCH_CONDA_CHANNEL}"
-    if [[ ${TARGET_OS} = "windows" ]]; then
+    if [[ ${TARGET_OS:-} = "windows" ]]; then
         AWS_ENABLED=0
     fi
 else