Fix threading issues in HLT prescale loading but for real

This MR is doing what !44936 (merged) claimed to do but missed the change in the most important two lines. 🤦 Namely:

[L1|HLT]PrescaleCondAlg: Use thread-safe insert to add elements to tbb::concurrent_unordered_map instead of operator[] + assignment which is unsafe.

!44936 (merged) did this only in two calls in initialize() of each algorithm, but missed the one that was actually causing crashes/errors which was in execute().

Fixes ATR-23670 though it's unfortunately impossible to test this due to how rare this race condition is. We'll deploy this at P1 ASAP and hope we don't see the errors ever again.

cc @mark, @astruebi, @palacino