Add NugetAudit msbuild settings (#2821)

Directory.Build.props

@@ -11,8 +11,20 @@
     <TargetsRoot>$(EngRoot)build/</TargetsRoot>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(TF_BUILD)' == 'true'">
-    <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
+  <PropertyGroup>
+    <ContinuousIntegrationBuild>false</ContinuousIntegrationBuild>
+    <ContinuousIntegrationBuild Condition="'$(TF_BUILD)' == 'true'">true</ContinuousIntegrationBuild>
+    <CI>$(ContinuousIntegrationBuild)</CI>
+  </PropertyGroup>
+
+  <PropertyGroup>
+    <!-- Nuget audit as warnings only, even in TreatWarningsAsErrors. -->
+    <!-- Except for in CI, critical will fail the build. -->
+    <WarningsNotAsErrors>$(WarningsNotAsErrors);NU1901;NU1902;NU1903</WarningsNotAsErrors>
+    <WarningsNotAsErrors Condition="'$(CI)' == 'false'">$(WarningsNotAsErrors)NU1904</WarningsNotAsErrors>
+    <WarningsAsErrors Condition="'$(CI)' == 'true'">$(WarningsAsErrors)NU1904</WarningsAsErrors>
+    <NuGetAuditLevel>moderate</NuGetAuditLevel> <!-- warn on moderate severity only. -->
+    <NuGetAuditMode>all</NuGetAuditMode> <!-- audit transitive dependencies. -->
   </PropertyGroup>
 
 </Project>