pmb.sideload: Remove irrelevant output when querying architecture

Previously, if this was the first time you were connecting to a device, the output of the ssh command would contain a warning about that the host was added to the list of trusted hosts. This causes problems as this gets included in the output where the code previously expected the string-representation of the architecture to be.

As such, remove the irrelevant output by splitting it into lines and only using the last line as we assume ssh won't print anything after the actual output of the invoked command.

Closes #2335 (closed)