Cut the CI build time: analyzers once per project, cache NuGet

[mgravell]

The .NET Build step was ~4 minutes on the Windows runner. Measured with /p:ReportAnalyzer=true across all 42 compilations of a clean build: analyzers are 226s of CPU and ~40% of wall time, of which StyleCop is 155s and PublicApiAnalyzers 37s. Our own analyzer and generators are 5.2s - about 2% - so they are not the problem. Restore is another ~80s, uncached on both runners.

• run analyzers on one TFM per project (the newest it builds) rather than once per TFM: every rule we run is TFM-agnostic, so a six-TFM library was paying for the same diagnostics six times. Generators are unaffected. /p:RunAnalyzers=true gets the full per-TFM sweep back.
• StyleCop applies to src/ only; it was buying nothing in tests/ and toys/ that review does not.
• cache ~/.nuget/packages in both CI jobs.
• --no-build on the CI test steps, which were each re-entering the build for ~15s.
• drop GeneratePackageOnBuild from RESPite.Benchmark, which packed a nupkg on every Release build; the pack step still publishes it.

Clean full build locally: 26.7s -> ~19s, Csc CPU 110-124s -> 70-83s.

Checklist

• I fully and freely contribute this code in accordance with the project license (and am legally able to do so)
• I take responsibility for this contribution's quality and correctness, including any portions produced with AI assistance (see CONTRIBUTING.md).