nixos manual: recommend use of dd for writing the image

Unetbootin works by altering the image and placing a boot loader on it.
For this reason, it cannot work with UEFI and the installation guides
for other distributions (incl. Debian and Fedora) recommend against
using it.

Since dd writes the image verbatim to the drive, and not just the files,
it is not necessary to change the label after using it for UEFI
installations.

vcunat: tiny changes to the PR. Close #14139.