Allow overriding element.bst files with users buildstream.conf file

To allow running epiphany in a buildstream build shell, I added a bunch of meson goo to compile gschemas in the build directory, and added this snippet at the top of main:

#if DEVELOPER_MODE
  g_setenv ("GSETTINGS_SCHEMA_DIR", BUILD_ROOT "/data", FALSE);
#endif

Of course, we don't want to set this environment variable except in developer builds, so it needs to be contingent on the -Ddeveloper_mode=true meson option.

Problem is there's no easy way to tell BuildStream to set this option when building epiphany, except by modifying the element file core/epiphany.bst. That's undesirable because that file is versioned. I want this to be a local change only.

In JHBuild the way to do this was to add a snippit like this to ~/.config/jhbuildrc:

module_mesonargs['epiphany'] = '-Ddeveloper_mode=true'

which made it very easy to have local overrides for project settings. Being able to do this with BuildStream would be highly desirable.