Fix basebuild job by adding multiarch support (was trying to build on x86_64 with an arm64 image)

Change-Id: Ie9be1a1a7aac016139e17d3293e22e744d527eaa
diff --git a/packer/vars/centos-7.json b/packer/vars/centos-7.json
index 5e6d1c8..aa629a9 100644
--- a/packer/vars/centos-7.json
+++ b/packer/vars/centos-7.json
@@ -4,5 +4,6 @@
   "ssh_user": "centos",
 
   "distro": "CentOS 7",
+  "arch": "x86_64",
   "cloud_user_data": "provision/rh-user_data.sh"
 }