Shorten starship nix_shell segment to stop narrow-window truncation

Fish truncates an overlong prompt line from the left with an ellipsis, hiding the directory. In nix develop / nix-shell the default nix_shell format adds 'impure (nix-shell-env)' (~22 extra chars). Setting nix_shell.format = "via [$symbol]($style)" keeps only the icon. When editing starship.nix symbols via sed, note that Nerd Font glyphs are multibyte private-use characters, so pattern-matching on the literal symbol often fails; anchor on line numbers or the key name instead.