install to-existing-root: don't blow away /boot until later

In `install to-existing-root`, we can delay blowing away `/boot` until very late in the process - specifically until we've pulled the container image for example, which may hit problems.

It'd make us overall more robust to being `ctrl-c`'d in general and we could even have a last minute extra warning + delay there as it's really that process that's the destructive thing.

Rastrea el flujo de `install to-existing-root` e identifica dónde se descarga la imagen del contenedor y dónde se elimina `/boot`. Haz que el paso destructivo ocurra únicamente después de que la descarga de la imagen se complete correctamente, y verifica que una interrupción antes de ese momento no elimine `/boot`; considera la advertencia final y el retraso propuestos.

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