nice: Permit --adjustment to be specified multiple times

This is tested by the GNU coreutils test suite, and the
correct behaviour is the last specification wins.

Signed-off-by: Ed Smith <ed.smith@collabora.com>

src/uu/nice/src/nice.rs

@@ -190,6 +190,8 @@ pub fn uu_app() -> Command {
                 .short('n')
                 .long(options::ADJUSTMENT)
                 .help("add N to the niceness (default is 10)")
+                .action(ArgAction::Set)
+                .overrides_with(options::ADJUSTMENT)
                 .allow_hyphen_values(true),
         )
         .arg(