When making a bundle project, building fails when InstallerRuntimeIdentifiers is manually set inside.

- [ ] This issue is blocking
- [x] This issue is causing unreasonable pain

For some reason I been hitting this issue when experimenting with making an installer bundle similar to those found in dotnet/runtime using the arcade package Microsoft.DotNet.Build.Tasks.Installers.

The issue happens on this line:

https://github.com/dotnet/arcade/blob/943d03f62955c771825dfa1f1bdeb8f853a2d7dd/src/Microsoft.DotNet.Build.Tasks.Installers/build/installer.targets#L85-L86

The following is what is in my bundleproj file:

```xml

net6.0


Elskom Shared Framework Bundle Installer
$(MSBuildProjectDirectory)
elskom-runtime
elskom-runtime
win-x86;win-x64;win-arm64;osx-x64;osx-arm64
Runtime
Elskom
$(MSBuildProjectDirectory)/shared-framework-distribution-template-$(TargetArchitecture).xml
com.elskom.dotnet.Elskom.Sdk.App.$(ProductVersion).osx.$(TargetArchitecture)
osx_resources

```

Perhaps that line can be made to not error when ``InstallerRuntimeIdentifiers`` is set, it seems to also happen if I set ``RuntimeIdentifiers`` only as well.

I been doing this sort of experiments with the arcade stuff to learn more about what was done to set up creating them for the normal .NET SDK / Runtime installers in the first place. When I done this I expected it to just work like I see with the runtime repository. Perhaps I am missing something I have not seen yet from the runtime repository which might explain why this just fails for some reason.