ci: fix convert test for elastic dockerfile

elastic/dockerfiles recently removed the actual dockerfiles from main,
so now you have to go to a specific branch to get it

Signed-off-by: Michael McCracken <mikmccra@cisco.com>

Author: mikemccracken

test/convert.bats

@@ -72,7 +72,7 @@ EOF
 
 @test "elasticsearch" {
   skip_slow_test
-  git clone https://github.com/elastic/dockerfiles.git
+  git clone -b v6.8.8 https://github.com/elastic/dockerfiles.git
   chmod -R a+rwx dockerfiles
   cd dockerfiles/elasticsearch
   stacker convert --docker-file Dockerfile --output-file stacker.yaml --substitute-file stacker-subs.yaml