enable referencing source link packages only if GITHUB_ACTIONS is true

Author: smdn

src/Smdn.MSBuild.ProjectAssets.Library/project/Project.props

@@ -68,7 +68,12 @@ SPDX-License-Identifier: MIT
     source link
   -->
   <ItemGroup>
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All"/>
+    <PackageReference
+      Include="Microsoft.SourceLink.GitHub"
+      Condition="'$(GITHUB_ACTIONS)' == 'true'"
+      Version="1.1.1"
+      PrivateAssets="All"
+    />
   </ItemGroup>
 
   <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true'">