fix ios build failed issue

[jonathanz]

cp -r behavior is different when the target path exists or not. It will cause the sign step failed.
cp -r ~/files ~/files-backup
If the directory files-backup already exists, the directory files will be placed inside.
If files-backup does not already exist, it will be created and the contents of the files directory will be placed inside it.

[jaimecbernardo]

Thank you for the contribution.
Didn't have this issue ever occur to me, but I see no harm in the proposed fix.
Out of curiosity, on what version of Cordova did this occur? Or was this on another framework, such as Ionic?