Switch from privileged to unprivileged on DEB/RPM

[michalpristas]

In a current codebase we assume service path to be located at /etc/sytemd/system...

elastic-agent/internal/pkg/agent/application/upgrade/service_update_linux.go

Line 26 in 99a477c

unitFilePath := "/etc/systemd/system/" + paths.ServiceName() + ".service"

elastic-agent/internal/pkg/agent/install/svc_linux.go

Line 22 in 99a477c

serviceFilePath := fmt.Sprintf("/etc/systemd/system/%s.service", paths.ServiceName())

elastic-agent/internal/pkg/agent/install/svc_linux.go

Line 53 in 99a477c

serviceFilePath := fmt.Sprintf("/etc/systemd/system/%s.service", serviceName)

in kardianos as well
https://github.com/kardianos/service/blob/b7ded6fcb7497ba4f88edf9cc3445411175b8897/service_systemd_linux.go#L79

but our package spec for deb says /lib/systemd/system/...

elastic-agent/dev-tools/packaging/packages.yml

Line 300 in 99a477c

/lib/systemd/system/{{.BeatServiceName}}.service:

this breaks unprivileged/privileged subcommands in a way

Error: failed to read service file /etc/systemd/system/elastic-agent.service: open /etc/systemd/system/elastic-agent.service: no such file or directory

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)