bug(install): not error out when a is not a dir in -d

Hi, uutils mainteners we confirmed the install uu utility it do not error out at all when a is not a dir, as i confirmed ```css relunsec@relunsec:~/software/coreutils/target/debug$ ./install -d a a relunsec@relunsec:~/software/coreutils/target/debug$ gnuinstall -d a a gnuinstall: cannot create directory ‘a’: File exists gnuinstall: cannot create directory ‘a’: File exists relunsec@relunsec:~/software/coreutils/target/debug$ ```