workflows/release-binaries: Add missing output variable

Reviewed By: thieta

Differential Revision: https://reviews.llvm.org/D148734

(cherry picked from commit cd39791)

Author: tstellar

.github/workflows/release-binaries.yml

@@ -30,6 +30,7 @@ jobs:
       build-dir: ${{ steps.validate-tag.outputs.build-dir }}
       rc-flags: ${{ steps.validate-tag.outputs.rc-flags }}
       ref: ${{ steps.validate-tag.outputs.ref }}
+      upload: ${{ steps.validate-tag.outputs.upload }}
 
     steps:
     - name: Checkout LLVM