builder-module: Fix cmake libdir value

Fixes 8c036e0

It is being set to `$prefix/lib` but CMAKE_INSTALL_LIBDIR is not an
absolute path by convention [1]:

> It should be a path relative to the installation prefix

When it is set to `/app/lib`, it is causing issues in applications [2]

[1]: https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html#result-variables

[2]: flathub/io.github.martinrotter.rssguard#73 (comment)

src/builder-module.c

@@ -1788,7 +1788,7 @@ builder_module_build_helper (BuilderModule  *self,
           if (libdir)
             g_ptr_array_add (configure_args_arr, g_strdup_printf ("-DCMAKE_INSTALL_LIBDIR:PATH='%s'", libdir));
           else
-            g_ptr_array_add (configure_args_arr, g_strdup_printf ("-DCMAKE_INSTALL_LIBDIR:PATH='%s/lib'", prefix));
+            g_ptr_array_add (configure_args_arr, g_strdup_printf ("-DCMAKE_INSTALL_LIBDIR:PATH='lib'"));
           g_ptr_array_add (configure_args_arr, g_strdup ("-G"));
           g_ptr_array_add (configure_args_arr, g_strdup_printf ("%s", cmake_generator));
         }