sideload: doas retake
The previous attempt in 099a7741 spawned two additional SSH connections to decide whether to use doas or sudo, this adds a lot of overhead for something relatively simple, and didn't work correctly on my device (it tried using doas even though I have sudo).
Let's rework this to figure out the right command on the fly, preferring sudo (or sudo-doas-wrapper) and falling back to doas if it's not available.