toggling --enable-profiling should not do a full rebuild

If the developer wishes to profile their app, a rebuild is necessary. But when finished with their profiling session, the developer typically wishes to return to their regular build flags. The current behaviour is that another full rebuild will take place at this point.

If the developer is working on performance improvements, where they must benchmark with optimisations and investigate with profiling, this incurs 2 full rebuilds on every cycle. A workaround is to have two checkouts, but it is very confusing to work like that.

It would be a very good boost to developer productivity if the `--enable-profiling` toggle used a different binary cache directory, such that every cycle only recompiled what was necessary for the profiled, or optimised, binaries, respectively.