fix: fix reference used for inline linear SCC diffcache

[AayushSabharwal]

PreallocationTools.get_tmp needs a value passed to it to know whether to return a dual buffer or not. Previously, findfirst(!SU.isconst, A) ignored the fact that A isa Matrix{Num} and needs unwrap. However, !isconst will still find parameters, which won't be duals even if unknowns are. iscall suffers from the same problem. Using search_variables! and/or query to find entries of A/b with unknowns is expensive. This approach prefers to find a differential variable and use that. The downside is that this may use a dual buffer even if A or b don't have duals, but shouldn't be too bad.