agnoster: use %n instead of $USER to fix quoting

Fixes #7268

With `$USER`, we'd need to quote it in case special characters like `\` are present in
the $USER value, like if the user is part of an AD domain.

With `%n` the quoting is done automatically by zsh.
See http://zsh.sourceforge.net/Doc/Release/Prompt-Expansion.html#Login-information