Use Base.promote_op() instead of Base._return_type()

return_type() is kinda broken in v1.10, see:
JuliaLang/julia#52385

In any case Base.promote_op() is the official public API for this operation so
we should use it anyway.

Author: JamesWrigley

Diff for: src/submission.jl

@@ -186,7 +186,7 @@ end
 function EagerThunkMetadata(spec::EagerTaskSpec)
     f = chunktype(spec.f).instance
     arg_types = ntuple(i->chunktype(spec.args[i][2]), length(spec.args))
-    return_type = Base._return_type(f, Base.to_tuple_type(arg_types))
+    return_type = Base.promote_op(f, arg_types...)
     return EagerThunkMetadata(return_type)
 end
 chunktype(t::EagerThunk) = t.metadata.return_type