Fix cecil strongname build failure with arcade

Work around dotnet/arcade#2321

external/Mono.Cecil.overrides

@@ -1,13 +1,24 @@
 <Project>
 
+  <!-- Cecil's Directory.Build.Props imports this file if it
+       exists. We use it to import our own Directory.Build.props. -->
+  <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))" />
+
+  <PropertyGroup>
+    <!-- Cecil sets PublicSign on windows, but we always want to
+         PublicSign. -->
+    <PublicSign>true</PublicSign>
+  </PropertyGroup>
+
   <PropertyGroup Condition=" '$(ArcadeBuild)' == 'true' ">
+    <!-- Work around
+         https://github.com/dotnet/arcade/issues/2321. This disables
+         arcade's selection of AssemblyOriginatorKeyFile based on the
+         StrongNameKeyId, since we want to use cecil's key file. -->
+    <StrongNameKeyId></StrongNameKeyId>
+    <PublicKey>unknown</PublicKey>
+    <PublicKeyToken>unknown</PublicKeyToken>
     <NoWarn>$(NoWarn);0618</NoWarn>
-    <!-- Signing properties from cecil are overridden by Arcade
-         because cecil imports the .NET SDK (and hence Arcade) in a
-         nonstandard way, so set them again here. -->
-    <SignAssembly>true</SignAssembly>
-    <PublicSign>true</PublicSign>
-    <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)cecil/cecil.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
 
-</Project>
+</Project>