This is not a real fix. I still haven't figured out why a `connection
reset by peer` happens in the first place. `target.Reboot()` will still
block for a bit before giving up. At least it doesn't falsely report a
fail now.
Also moved canc() up so the terminal isn't littered with `[update boot
file system] 0.00% of 16 MiB, uploading at 0 B/s` during the wait.
Related: https://github.com/gokrazy/gokrazy/issues/199
