resources: Revert "Remove the 'arm64-ubuntu-18.04-img'..."

This reverts commit 03017fe4afbca58c04ec697b5b6d6e73921b4cbf.
https://gem5-review.googlesource.com/c/public/gem5-resources/+/59530

Change-Id: If6317d1e96af678ed32916496dd5e8922d7ebaae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/59929
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/resources.json b/resources.json
index 0da6214..c8b6778 100644
--- a/resources.json
+++ b/resources.json
@@ -144,6 +144,19 @@
         },
         {
             "type": "resource",
+            "name" : "arm64-ubuntu-18.04-img",
+            "documentation" : "A disk image containing Ubuntu 18.04 for arm64.",
+            "architecture" : "ARM",
+            "is_zipped" : true,
+            "md5sum" : "ec7d0156366743fa7b93cbfcea82925c",
+            "url" : "{url_base}/images/arm/ubuntu-18-04/arm64-ubuntu-20220425.img.gz",
+            "source" : null,
+            "additional_metadata" : {
+                "root_partition": "1"
+            }
+        },
+        {
+            "type": "resource",
             "name" : "arm64-ubuntu-20.04-img",
             "documentation" : "A disk image containing Ubuntu 18.04 for arm64 which will run an `m5 readfile` instruction after booting. If no script file is specified an `m5 exit` instruction will be exected.",
             "architecture" : "ARM",