use arithmetics in curl_retry_on_18 while loop

((attempts++)) evaluates to 1 on first call, causing an error with set -o errtrace

bin/util/common.sh

@@ -121,8 +121,7 @@ export_env_dir() {
 curl_retry_on_18() {
 	local ec=18;
 	local attempts=0;
-	while [[ $ec -eq 18 && $attempts -lt 3 ]]; do
-		((attempts++))
+	while (( ec == 18 && attempts++ < 3 )); do
 		curl "$@" # -C - would return code 33 if unsupported by server
 		ec=$?
 	done