When `copyFiles` is used to copy read-only files, it now makes the copy writable.

Previously, if the target file would already exist due to a prior run of Dub, `copyFiles` would produce an access
denied error because the read-only target could not be overwritten. Note that if you were affected by this behaviour,
you will need to remove those files by hand once to eliminate these errors.

It is common for version control systems to mark binary files read-only in the working copy, to prevent concurrent
edits of files in unmergeable formats.
