chore: support local NuGet source via NOSCORE_LOCAL_PACKAGES env var (#737)

Move the wiring out of NuGet.config (where %NOSCORE_LOCAL_PACKAGES%
fails NU1301 when unset) into a conditional Directory.Build.props
PropertyGroup. RestoreAdditionalProjectSources is only set when the
env var is defined, so CI and fresh clones are unaffected.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwan-joly

Directory.Build.props

@@ -0,0 +1,5 @@
+<Project>
+  <PropertyGroup Condition="'$(NOSCORE_LOCAL_PACKAGES)' != ''">
+    <RestoreAdditionalProjectSources>$(RestoreAdditionalProjectSources);$(NOSCORE_LOCAL_PACKAGES)</RestoreAdditionalProjectSources>
+  </PropertyGroup>
+</Project>

NuGet.config

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <configuration>
     <packageSources>
-        <add key="local" value="%NOSCORE_LOCAL_PACKAGES%" />
         <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     </packageSources>
-</configuration>
+</configuration>