build: fix TestS3Minio by switching to pgsty/minio as quay.io image is gone

MinIO no longer publishes images on quay.io (nor Docker Hub or ghcr.io)
so pulling quay.io/minio/minio returns "unauthorized". pgsty/minio is a
maintained community fork with the same entrypoint layout.

fstest/testserver/init.d/TestS3Minio

@@ -14,7 +14,7 @@ start() {
            -e "MINIO_ACCESS_KEY=$USER" \
            -e "MINIO_SECRET_KEY=$PASS" \
            -p 127.0.0.1:${PORT}:9000 \
-           quay.io/minio/minio server /data
+           pgsty/minio server /data
     
     echo type=s3
     echo provider=Minio