ci: avoid heredoc in unit tests coverage export

Jenkinsfile

@@ -124,12 +124,7 @@ spec:
               coverage="$(go tool cover -func=build/coverage.out | awk '/^total:/ {gsub("%","",$3); print $3}')"
             fi
             export GO_COVERAGE="${coverage}"
-            python3 - <<'"'"'PY'"'"'
-import json, os
-coverage = float(os.environ.get("GO_COVERAGE", "0") or "0")
-with open("build/coverage.json", "w", encoding="utf-8") as handle:
-    json.dump({"summary": {"percent_covered": coverage}}, handle)
-PY
+            printf '{"summary":{"percent_covered":%s}}\n' "${GO_COVERAGE}" > "${COVERAGE_JSON}"
             exit ${test_rc}
           '''
         }