Only run analyzers in netcoreapp3.1 build

[RikkiGibson]

/cc @jaredpar @sharwell

One thing that came up is the most expensive analyzer is code style. Now that we're multi-targeting we're essentially paying that cost twice: once per target framework. Why not condition that to only run on one TF instead of both?

The logic already exists to make this distinction with nullable reference types. Essentially we only enable NRT for a specific TF, not both. We could consider hooking into that for the style based analyzers as well.

[sharwell]

I see no issue with this for code style analyzers.

We should keep our correctness analyzers in all the builds though.