🐛 systemd timer is only started for the current boot

**Describe the bug**
Only `systemd start` is called on the timer Unit during installation, so it might not be enabled after reboot.

https://github.com/cloudflare/cloudflared/blob/f27418044ba359a17995166034d900c11aa50008/cmd/cloudflared/linux_service.go#L308

**To Reproduce**
N/A. See source code.

**Expected behavior**
Call `systemd enable --now` to enable the timer and start it immediately.

**Environment and versions**
- OS: Linux
- Architecture: AMD
- Version: 2024.4.1

**Logs and errors**
N/A

**Additional context**
N/A