Microsoft.Net.Compilers is only supported on MSBuild v15.0 and above
whatever the auto detect tooling is for KUDU, it always selects 14
is
The msbuild 15 is now available in Azure, however it's not default you need to add a custom deployment script to override Kudu's project detection logic .
As watashiSHUN described you could add something like this:
nuget restore "%DEPLOYMENT_SOURCE%\{SolutionPath}"
"%MSBUILD_PATH%" {MSBuildArguments}
And here is the template and the issue in the github.