fsc on .NET 6/7 is spawning `dotnet` process

We noticed that every invocation of `fsc` on .NET 6/7 is causing a spawn of dotnet to find the installed .NET SDKs, in order to determine which assembly references are "system" assemblies.

One attempt to fix that is here #13863 

We should also prioritise a simpler fix as this is causing unnecessary loss of compiler perf.