Fix Pack to respect NoBuild property for reference assemblies

[Copilot]

msbuild /t:pack /p:NoBuild=true ignores the NoBuild flag when reference assemblies are used, causing unwanted rebuilds that overwrite signed DLLs.

Root Cause

The _SdkGetReferenceAssemblies target unconditionally depends on Build, forcing reference assembly projects to rebuild during pack operations regardless of the NoBuild property.

Changes

Modified Source/MSBuild.Sdk.Extras/Build/ReferenceAssembly.targets:

• Introduced _SdkGetReferenceAssembliesWithBuild as a conditional build gate:

  <Target Name="_SdkGetReferenceAssembliesWithBuild" 
          DependsOnTargets="Build" 
          Condition="'$(NoBuild)' != 'true'" />

• Updated _SdkGetReferenceAssemblies to depend on the conditional target instead of Build directly

• Modified _ExtrasGetRefsForPackage to propagate NoBuild=$(NoBuild) to reference assembly project invocations

When NoBuild is true, the target executes but skips the Build dependency, using existing outputs from disk.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Pack always builds a project with reference assemblies</issue_title>
<issue_description>1. Add a reference assembly to a project, like described in the blog (e.g. https://github.com/AzureAD/microsoft-authentication-library-for-dotnet)

2. Build the sln, which builds both project and ref project
3. Pack via "msbuild /t:pack /p:nobuild=true"

Actual:
The pack does not obey the "nobuild" setting. It needs to rebuild because the ref assemblies are not actually produced in step 2

Expected:
Pack should not rebuild

Impact:
Because pack rebuilds, some DLLs are overwritten. This makes DLL signing difficult. Ideally all DLLs should be on disk and /t:pack just packs

Workaround:
I can workaround by packing twice. So my release pipeline looks like this:

• build
• pack
• sign lib assemblies
• sign ref assemblies
• pack /nobuild

</issue_description>

Comments on the Issue (you are @copilot in this section)

@clairernovotny I'll look into this when I can, but in general, I would not suggest making sign as part of the build step. I'd have build/pack do just that, then send the nuget to the signing mechanism once built (you can always extract/repack the nuget with zip before the nuget itself is signed).

• Fixes Pack always builds a project with reference assemblies #136

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