The 2b-agent provisioner installed agent.env from /tmp/etc-stage, but the 2a
package step deletes /tmp/etc-stage before 2b runs, so the build failed with
"cannot stat '/tmp/etc-stage/fleet-agent/agent.env'". 2b-hostapi avoids this by
installing from its own upload dir; do the same: move agent.env into files/agent/
so it rides the agent's file upload (/tmp/agent), which survives until 2b-agent
cleans it. Runtime path (/etc/fleet-agent/agent.env) is unchanged.
