Fix SimpleCI to use different data directory than mint (#7520)

Currently, the backend minio server uses the same data directory
as the mint test itself, causing `s3 sync` to fail often.

Now `minio` backend will use a different data directory `/data`
instead of `/mint/data`

buildscripts/gateway-tests.sh

@@ -22,7 +22,7 @@ set -o pipefail
 function start_minio_server()
 {
     MINIO_ACCESS_KEY=minio MINIO_SECRET_KEY=minio123 \
-                    minio --quiet --json server data --address 127.0.0.1:24242 > server.log 2>&1 &
+                    minio --quiet --json server /data --address 127.0.0.1:24242 > server.log 2>&1 &
     server_pid=$!
     sleep 3