Fixes #6762

I messed up in #6706 and forgot to check that the archives were at the correct place (and to read the man page correctly).
In 2.5.0~alpha1 the archives are in the tarball (which i did check) but they were in the toplevel directory instead of src_ext/archives.

Per git-archive(1):

--prefix=<prefix>/
    Prepend <prefix>/ to paths in the archive. Can be repeated; its rightmost value is used for all
    tracked files. See below which value gets used by --add-file.

--add-file=<file>
    Add a non-tracked file to the archive. Can be repeated to add multiple files. The path of the
    file in the archive is built by concatenating the value of the last --prefix option (if any)
    before this --add-file and the basename of <file>.

So a --prefix needs to be inserted before each --add-file and the "main" prefix needs to be moved to be the last argument as it will be used for all the other files.

Backported to the 2.5 branch in #6766