Since `IDOverrideLibraryProperty` apparently stores its rna_path with
string-based collection keys, we have to make sure we also use string-
based keys elsewhere in `OverrideRNAPathTreeBuilder::build_path` and
down the line, otherwise:
- we get duplicate collection entries ("pose.bones[1].constraints" vs.
"pose.bones["Bone.001"].constraints")
- crashes may occur (we are reading on an already freed `TreeElement`
see #118307 for ASAN output)
So now make sure we are using string-based collection keys when we can
so the TreeElements match /and dont get duplicated).
NOTE: the duplication of entries came with 67b92418ee
Philipp Oeser