Log version info in Net2SlowTaskTrace (and other backtraces?)

The `Net2SlowTaskTrace` event includes backtraces, and it would probably be convenient to include version information such that we could more readily symbolicate them. We could also consider doing this in other locations that we log backtraces.

I would have already done this, but with the dual build systems it was actually a bit trick to get access to the version information in the location that I needed it. As a result, this is probably best deferred until the Make build system has been removed.