chg: Modernize CI workflow, enable SourceLink

- Bump workflow actions to latest majors (Node 24); drop deprecated softprops/action-gh-release@v1
- Trigger push builds on main instead of master
- Create local nuget feed dir before pack (fixes NU1301)
- Enable Microsoft.SourceLink.GitHub for debugging symbols (ref PR #340)

Raylib-cs/Raylib-cs.csproj

@@ -35,6 +35,10 @@
         <None Include="Raylib-cs.targets" Pack="true" PackagePath="buildTransitive\Raylib-cs.targets"/>
     </ItemGroup>
 
+    <ItemGroup>
+        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" PrivateAssets="All"/>
+    </ItemGroup>
+
     <!-- Import Build Props -->
     <Import Project="./Build.props" Condition="Exists('./Build.props')"/>
     <PropertyGroup Condition="'$(TargetRaylibTag)' == ''">