Warn on invalid NuGet configuration (#10220)

Issue a warning when `%NUGET_PACKAGES%` doesn't have a trailing slash
until the associatede issue is fixed.

dotnet/roslyn#72657

.gitignore

@@ -24,8 +24,9 @@ x86/ !eng/common/cross/x86/
 msbuild.log
 msbuild.err
 msbuild.wrn
-msbuild.binlog
 msbuild.ProjectImports.zip
+*.binlog
+*.complog
 
 # Visual Studio 2015
 .vs/

Directory.Build.targets

@@ -34,6 +34,16 @@
     <SupportedPlatform Remove="iOS" />
   </ItemGroup>
 
+  <!--
+    Warn consumers until the underlying issue is fixed
+    https://github.com/dotnet/roslyn/issues/72657
+  -->
+  <Target Name="CheckNuPkgEnvEndsWithSlash"
+          AfterTargets="AfterCompile">
+    <Warning Condition="'$(NUGET_PACKAGES)' != '' AND !$(NUGET_PACKAGES.EndsWith($([System.String]::new($([System.IO.Path]::DirectorySeparatorChar)))))"
+             Text="NUGET_PACKAGES should end with a slash or it will lead to editorconfig issues: $(NUGET_PACKAGES)" />
+  </Target>
+
   <Target Name="GetCustomAssemblyAttributes"
           BeforeTargets="GetAssemblyAttributes"
           Condition=" '$(MSBuildProjectExtension)' == '.csproj' "