[flang] Remove double pointer indirection for _QQEnvironmentDefaults (#90615)

A double pointer was being passed to the call to FortranStart rather than just a pointer to the EnvironmentDefaults.list. This now passes `null` directly when there's no EnvironmentDefaults.list and passes the list directly when there is, removing the original global variable which was a pointer to a pointer containing null or the EnvironmentDefaults.list global.

Fixes #90537