Follow up to #34782

Build.cmd in the root of the runtime repo lets random values on several parameters pass and actually start the building process, only to fail much later with unrelated, and therefore misguiding errors. It is worth highlighting this happens both, on Windows and Linux. Running various cases, the flags that allow these "garbage" parameters are:

• Configuration
• OS
• Architecture

I've started working on this, but there are other things that remain unclear, which are preventing me of making further progress.

• Does build.cmd support any platform other than Windows? The -help says the possible options are Windows_NT and Unix, but the latter fails. This help documentation might need to be updated.
• On the other hand, build.sh lists a whole different (and bigger) list of supported platforms: Windows, Linux, FreeBSD, OSX, tvOS, iOS, Android, WebAssembly. Can I assume Windows is wrong here as build.cmd does that?

A funny story that added to my motivation on working on this fix, is that I spent so much time one day trying to build runtime only to fail over and over again. It turns out I had a typo in one of those flags.