CI doesn't build `net9.0` targets in source generators.

[eiriktsarpalis]

A while back we added net9.0 targets to the source generator projects so that they support nullability annotations. These aren't being used by the product but help with local development. We discovered with #100345 that these targets aren't being built by CI, so it's easy for them to be broken.

cc @ViktorHofer

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[huoyaoyuan]

My local development environment lacks SG in VS currently, which is annoying when developing interop-heavy code. Sounds related to this?

[ericstj]

Shouldn't be. These SG builds aren't meant to be run anywhere. Just built to validate nullable annotations.

If SG's aren't working in your dev environment it's probably because you're using a VS that's older than the compiler toolset we're using and they are failing to load. Make sure you're on a preview VS with the 9.0 preview SDK installed.