Fix that CI builds sometimes produce different analyzer violations than local builds (OSOE-855)

[Piedone]

See e.g. this most recent example: Lombiq/Helpful-Libraries#261. I also started an investigation there.

Jira issue

[Piedone]

My minimal repro still surfaces the issue under .NET v8.0.204: https://github.com/Lombiq/OSOE-855-AnalyzersTest/actions/runs/9096693522/job/25002706433

[Piedone]

Opened Roslyn issues:

• "IDE0305 Use collection expression for fluent" not raised if ToArray() is assigned to a var variable dotnet/roslyn#73638
• "IDE0305 Use collection expression for fluent" inconsistent between .NET 8.0.204 and 8.0.300 despite explicit analyzer package dependency dotnet/roslyn#73639

[Piedone]

We need to pin the .NET version number: dotnet/roslyn#73639 (comment)