Fix nightly check in CI release build

.github/workflows/release.yml

@@ -31,7 +31,7 @@ jobs:
             - name: Compute release name and tag
               id: release_info
               run: |
-                  if [[ $IS_NIGHTLY ]]; then
+                  if $IS_NIGHTLY; then
                     echo "::set-output name=tag_name::nightly-${GITHUB_SHA}"
                     echo "::set-output name=release_name::Nightly ($(date '+%Y-%m-%d'))"
                   else