Fix ISRG root cert URL and file name

buildbot/buildbot_steps.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-LE_CERT=isrg-root-x1.pem,isrg-root-x2.pem
+LE_CERT=isrgrootx1.pem,isrg-root-x2.pem
 
 function clone_ipxe_upstream() {
     local ipxe_src_dir="$1"
@@ -49,7 +49,7 @@ function sed_enabled_ipxe_features() {
 
     echo "Downloading default iPXE CA certificate"
     cd ipxe/src
-    wget https://letsencrypt.org/certs/isrg-root-x{1,2}.pem
+    wget https://letsencrypt.org/certs/{isrgrootx1,isrg-root-x2}.pem
 
     return 0
 }