The UI code for nodes uses `stringstream`s a few times to format
strings. In some cases it is fine, but in others it makes localization
hard or impossible to do properly, as explained in #92758.
This commit replaces some `stringstreams` by `fmt::format()`, a
library that is already used in other parts of Blender and was
suggested by Hans Goudey (@hooglyboogly) in
[D15996](https://archive.blender.org/developer/differential/0015/0015996/#inline-138440).
Damien Picard