build: ensure cross-platform UTC timestamp generation

Author: kgryte

.github/workflows/demo.yml

@@ -143,7 +143,7 @@ jobs:
             --arg name '${{ env.ARRAY_LIBRARY }}' \
             --arg version "${library_version}" \
             --arg api_version '${{ env.ARRAY_API_VERSION }}' \
-            --arg timestamp "$(date --utc +'%Y-%m-%dT%H:%M:%SZ')" \
+            --arg timestamp "$(date -u +'%Y-%m-%dT%H:%M:%SZ')" \
             --arg platform "${platform}" \
             --arg system "${system}" \
             --arg release "${release}" \