Reference runtime variable

Author: NikolaMilosavljevic

eng/common/templates/vmr-build-pr.yml

@@ -31,5 +31,5 @@ stages:
 - template: /eng/pipelines/templates/stages/vmr-build.yml@vmr
   parameters:
     isBuiltFromVmr: false
-    isSourceOnlyBuild: $[variables['isSourceOnlyBuild']]
+    isSourceOnlyBuild: $(isSourceOnlyBuild)
     scope: lite