[flang][bbc] Fix dangling reference to `envDefaults` (#82800)

The lowering bridge stores the evvironment defaults (passed to the
constructor) as a reference. In the call to the constructor in bbc, the
defaults were passed as `{}`, which creates a temporary whose lifetime
ends immediately after the call.

The flang driver passes a member of the compilation instance to the
constructor, which presumably remains alive long enough, so storing the
reference in the bridge is justified. To avoid the dangling reference,
create an actual object `envDefaults` in bbc.