pmb.sideload: Query architecture from foreign device

Instead of assuming the architecture of the foreign device to be what the user selected in pmbootstrap init, actually query the architecture from the device and use that.

This does mean that one extra ssh connection is necessary, which does slow down the procedure somewhat. However, I think that is worth the user experience improvement this brings.

Also, the deduction process can be skipped by manually specifying --arch in the sideload invocation should it fail, or if one really wants to skip that extra ssh roundtrip.

Related: #2317