Use ECR credential helper when ECR plugin is enabled

Author: petetomasik

packer/linux/conf/buildkite-agent/hooks/environment

@@ -109,6 +109,7 @@ fi
 if [[ "${BUILDKITE_ECR_POLICY:-}" != "none" && "${ECR_PLUGIN_ENABLED:-}" == "1" ]]; then
   export BUILDKITE_PLUGIN_ECR_LOGIN=1
   export BUILDKITE_PLUGIN_ECR_RETRIES=3
+  export BUILDKITE_PLUGIN_ECR_CREDENTIAL_HELPER=true
 
   # map AWS_ECR_LOGIN_REGISTRY_IDS into the plugin list format
   if [[ -n "${AWS_ECR_LOGIN_REGISTRY_IDS:-}" ]]; then

packer/windows/conf/buildkite-agent/hooks/environment

@@ -55,6 +55,7 @@ fi
 if [[ "${BUILDKITE_ECR_POLICY:-}" != "none" && "${ECR_PLUGIN_ENABLED:-}" == "1" ]]; then
   export BUILDKITE_PLUGIN_ECR_LOGIN=1
   export BUILDKITE_PLUGIN_ECR_RETRIES=3
+  export BUILDKITE_PLUGIN_ECR_CREDENTIAL_HELPER=true
 
   # map AWS_ECR_LOGIN_REGISTRY_IDS into the plugin list format
   if [[ -n "${AWS_ECR_LOGIN_REGISTRY_IDS:-}" ]]; then