This checks in gjs_context_eval() to see if the GJS_ENABLE_PROFILER
environment variable is set but no profiler has been created. In that
case, we run a profiler for the duration of the JS code evaluation.

This is to accommodate applications such as Polari that are written in JS
but ship their own loader that links to libgjs and calls
gjs_context_eval(). This way, they can be profiled without writing any
extra code in their loaders (though they will still need it if, for
example, they want to add a command line argument.)