Stage2 mono builds fail while building fsharp due to using R2R

[tmds]

The mono stage2 builds go looking for crossgen2 while building fsharp but that package doesn't get built with a mono runtime (because mono can't use the r2r format).

The PublishReadyToRun properties added in dotnet/fsharp#16860 should be made conditional to not apply when the vmr is built with --use-mono-runtime.

cc @ViktorHofer @MichaelSimons @omajid

[dotnet-issue-labeler]

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[tmds]

Fixed by dotnet/fsharp#17300.