Fix plugin install directory (INSTALL_PLUGINDIR not PLUGIN_INSTALL_DIR).
    
    From the log:
    
    CMake Warning:
      Manually-specified variables were not used by the project:
        CMAKE_Fortran_FLAGS_RELEASE
        INCLUDE_INSTALL_DIR
        LIB_INSTALL_DIR
        LIB_SUFFIX
        PLUGIN_INSTALL_DIR            << note
        SHARE_INSTALL_PREFIX
        SYSCONF_INSTALL_DIR
    
    This affected riscv64 builds because it was wrongly using
    /usr/lib/mariadb/plugin, but the problem seems as if it could apply to
    other architectures.