Don't try running net48 tests on Linux/macOS

Author: nblumhardt

test/Serilog.Sinks.File.Tests/Serilog.Sinks.File.Tests.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net48;net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net48</TargetFrameworks>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
     <GenerateDocumentationFile>false</GenerateDocumentationFile>
   </PropertyGroup>