fix(images): avoid wrong AMI could be selected for ubuntu focal (github-aws-runners#2214)

* Changed the source_ami_filter for ubuntu focal

With the previous filter you could also get following ami:
 - ubuntu-eks/k8s_1.18/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*
We are looking specifically for this type:
- ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*
Which is unintended, as far as I understand.

* another_commit

* packer fmt

Author: blevkovych

images/ubuntu-focal/github_agent.ubuntu.pkr.hcl

@@ -88,7 +88,7 @@ source "amazon-ebs" "githubrunner" {
 
   source_ami_filter {
     filters = {
-      name                = "*/ubuntu-focal-20.04-amd64-server-*"
+      name                = "*ubuntu/images/hvm-ssd/ubuntu-focal-20.04-amd64-server-*"
       root-device-type    = "ebs"
       virtualization-type = "hvm"
     }