`--ip` causes early failures even when it doesn't matter

[mkeeter]

$ humility --ip 'fe80::0c1d:8cff:fec0:e207%en7'` --archive=~/oxide/hubris/target/grapefruit-standalone/dist/default/build-grapefruit-standalone-image-default.zip manifest
error: Invalid value "fe80::0c1d:8cff:fec0:e207%en7" for '--ip <IP>': Could not find interface for en7

The IP address isn't needed for humility manifest, but we parse it proactively when building the Cli object (either from the CLI or as an environment variable), and bail if we can't find the interface.

[hawkw]

this is kind of a loose relative of #597, another Weird CLI Error due to eagerly parsing all the address/archive/dump arguments