[rel/18.7] Skip nupkg verifier in .NET product build mode (#16092)

The binding redirect verifier in verify-nupkgs.ps1 fails in the dotnet/dotnet
VMR build because source-built dependency packages have older AssemblyVersion
stamps than NuGet.org packages (e.g. System.Runtime.CompilerServices.Unsafe
ships as 6.0.0.0 from source-build but 6.0.3.0 from NuGet.org).

The newVersion of a bindingRedirect must equal the AssemblyVersion of the
DLL actually shipped beside the exe, so no single value can satisfy both
the VMR/source-build pipeline (dnceng-public) and the internal pipeline
(dnceng / NuGet.org-based) at the same time.

Back-port the same skip already present on rel/18.8 and main: gate the
_VerifyNuGetPackages target on DotNetBuild != 'true' so the VMR no longer
runs the verifier, while the internal pipeline (which produces the actually-
shipped packages) continues to validate the redirects.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Evangelink

eng/AfterSolutionBuild.targets

@@ -4,7 +4,13 @@
     <SourceBranchName Condition=" '$(SourceBranchName)' == '' ">dev</SourceBranchName>
   </PropertyGroup>
 
-  <Target Name="_VerifyNuGetPackages" AfterTargets="Pack" Condition=" '$(OS)' == 'Windows_NT' ">
+  <!--
+    Verify binding redirects and framework targeting in nupkgs.
+    Skip in .NET product build mode: source-built dependency packages have different assembly versions
+    than NuGet.org packages (e.g. System.Collections.Immutable 11.0.0-ci ships assembly version 9.0.0.0),
+    so the binding redirect check produces false positives.
+  -->
+  <Target Name="_VerifyNuGetPackages" AfterTargets="Pack" Condition=" '$(OS)' == 'Windows_NT' and '$(DotNetBuild)' != 'true' ">
     <Exec Command="powershell -NoProfile -NoLogo -ExecutionPolicy Bypass $(RepositoryEngineeringDir)\verify-nupkgs.ps1 -configuration $(Configuration) -versionPrefix $(versionPrefix) -currentBranch $(SourceBranchName)" />
   </Target>