Include `msbuild` bin from the flatpak for `mono6` SDK extension

Regarding this disclaimer:
> Although built with Mono 6, this Flatpak uses .NET 8 for building C# solutions in Godot projects. To (re)build a C# solution in the Godot editor, make sure that "Editor -> Editor Settings --> Mono --> Builds --> Build Tool" is set to "dotnet CLI", and not "MSBuild (Mono)", before (re)building the C# solution, otherwise Godot won't be able to find MSBuild. It should be set to "dotnet CLI" by default, but if it isn't, reset it by going to the editor settings as previously described.

It *should* be possible to include a `/app/bin/msbuild` executable since the flatpak for mono6 SDK does include msbuild, according to flathub/org.freedesktop.Sdk.Extension.mono6#6 being closed.

I'm not sure how exactly to do it, since they didn't include instructions, but you could ask in that linked issue.
Another option is try to see how it's done in apt and replicate it. It would probably be creating a executable shell script in bin that does essentially `mono MSBuild.dll "$@"` with a bunch of parameters (see my comment on the linked issue).
But maybe it's easier than that, so it's worth asking them directly.