Add target os variable to windows validation (#1244)

Author: atalman

.github/workflows/validate-domain-library.yml

@@ -109,6 +109,7 @@ jobs:
         export PACKAGE_TYPE="${{ matrix.package_type }}"
         export CHANNEL="${{ matrix.channel }}"
         export SMOKE_TEST="${{ inputs.smoke_test }}"
+        export TARGET_OS="windows"
         eval $SMOKE_TEST
   validate-macos:
     if:  (inputs.os == 'macos' || inputs.os == 'all')