Turn off full signing on non-Windows platforms (#566)

Full Signing doesn't work on some platforms (eg RHEL 9, which disables
RSA+SHA1 which Full Signing needs). Full Signing is also really on
required for .NET Framework support. It should not be needed on modern
.NET (5 and later).

More details at dotnet/runtime#65874

Author: omajid

src/referencePackages/Directory.Build.props

@@ -25,6 +25,7 @@
     <KeyFileDir>$(RepoRoot)src/referencePackages/snk/</KeyFileDir>
     <SignAssembly>true</SignAssembly>
     <PublicSign>true</PublicSign>
+    <FullAssemblySigningSupported Condition="('$(FullAssemblySigningSupported)' == '') and ('$(OS)' != 'Windows_NT')">false</FullAssemblySigningSupported>
 
     <StrongNameKeyId>Microsoft</StrongNameKeyId>