fix(ci): coverlet.msbuild 8.0.0 breaks our CI, revert to 6.0.4

[skwasjer]

Error Message:
   Expected a <System.ArgumentNullException> to be thrown, but found <System.TypeInitializationException>:
System.TypeInitializationException: The type initializer for 'Coverlet.Core.Instrumentation.Tracker.IbanNet.DependencyInjection.Autofac_7ca6213b-8a07-4b4e-9001-39177f0b5236' threw an exception. ---> System.TypeLoadException: Could not resolve type with token 01000062 from typeref (expected class 'System.Environment' in assembly 'System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a')

There seems to be a fix upstream already but this is not deployed to NuGet yet, so we have to use previous version.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud