Disasmo overrides project's `DefineConstants`

Since the `dotnet build` cmdline now includes `/p:DefineConstants=DISASMO`, this value is taken instead of any value defined in the csproj.

I like being able to detect Disasmo runs from my project and/or code, but maybe an alternate property would be better. We could then bridge that into `DefineConstants` ourselves if we want that.

Start by locating where the dotnet build command adds /p:DefineConstants=DISASMO, then reproduce it with a project that already defines constants in its csproj. Done means project-defined constants are preserved while Disasmo runs remain detectable; the issue leaves the preferred alternate property undecided.

Written by the indexing model from the issue text.