agent name uses %spawn instead of %n

`%spawn` is a new client-side interpolation to dedupe multiple agent names
run via `buildkite-agent start --spawn N`. `%n` was resolved server-side
instead of client-side, which was problematic in some cases.

Author: pda

packer/linux/conf/bin/bk-install-elastic-stack.sh

@@ -115,7 +115,7 @@ if [[ -n "${BUILDKITE_AGENT_TOKEN_PATH}" ]] ; then
 fi
 
 cat << EOF > /etc/buildkite-agent/buildkite-agent.cfg
-name="${BUILDKITE_STACK_NAME}-${INSTANCE_ID}-%n"
+name="${BUILDKITE_STACK_NAME}-${INSTANCE_ID}-%spawn"
 token="${BUILDKITE_AGENT_TOKEN}"
 tags=$(IFS=, ; echo "${agent_metadata[*]}")
 tags-from-ec2-meta-data=true

packer/windows/conf/bin/bk-install-elastic-stack.ps1

@@ -86,7 +86,7 @@ If ($null -ne $Env:BUILDKITE_AGENT_TOKEN_PATH -and $Env:BUILDKITE_AGENT_TOKEN_PA
 
 $OFS=","
 Set-Content -Path C:\buildkite-agent\buildkite-agent.cfg -Value @"
-name="${Env:BUILDKITE_STACK_NAME}-${Env:INSTANCE_ID}-%n"
+name="${Env:BUILDKITE_STACK_NAME}-${Env:INSTANCE_ID}-%spawn"
 token="${Env:BUILDKITE_AGENT_TOKEN}"
 tags=$agent_metadata
 tags-from-ec2-meta-data=true