mysql_install_db: assume builddir is dirname0

The assumption in the original commit for --builddir (648d3ce), was to assume that the without a --builddir, and when --srcdir is specified, that the builddir is the same as the srcdir. For out of tree builds, the only supported by CMake type, this isn't the case.

As mysql_install_db is in the builddir, we use dirname0 after a quick check that my_print_defaults is also located from dirname0.