prompt_install_mode also runs inside felis setup. Setup then goes on
to the Owner and edge setup, which need the control plane, so choosing
nano there always ended in a setup error.
Under felis setup the mode is now full before any prompt or default is
considered, and an explicit FELIS_INSTALL_MODE=nano stops with a
message pointing at deploy/bootstrap.sh. That leaves the felis setup
branch of acquire_nano_binary unreachable, so it goes.
install_embedded_binary stays, since the full install still uses it.
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