Hi Marc,
Thanks for the note! That file is actually a sort of cross-platform way of storing symbolic links. BuildMaster also includes all the binaries needed for agents on both Windows and Linux, including .net runtimes for all three. In order to save space we deduplicate identical binaries from the agents. We use a simple json file instead of proper symlinks because we use the same file on Windows/Linux and it's easier this way. All the paths in the file have historically used Windows path separators due to how we generate it, and we convert those in BuildMaster at runtime to the proper format.... or at least we were supposed to. At some point there was a regression added that caused this to not happen when updating from Linux.
Anyway, this will be fixed as BM-3981 in the next release :)