Disable implicit NuGet fallback folder

In order to try to fix this error seen on the GitHub action:

> /home/runner/work/serilog-formatting-log4net/serilog-formatting-log4net/src/Serilog.Formatting.Log4Net.csproj : error NU1403: Package content hash validation failed for Microsoft.NETCore.Platforms.1.1.0. The package is different than the last restore. [/home/runner/work/serilog-formatting-log4net/serilog-formatting-log4net/serilog-formatting-log4net.sln]

See NuGet/Home#8574 and https://github.com/NuGet/docs.microsoft.com-nuget/issues/1998

Author: 0xced

src/Serilog.Formatting.Log4Net.csproj

@@ -32,6 +32,7 @@
   </PropertyGroup>
 
   <PropertyGroup Label="Restoring">
+    <DisableImplicitNuGetFallbackFolder>true</DisableImplicitNuGetFallbackFolder>
     <NuGetLockFilePath>../packages.lock.json</NuGetLockFilePath>
     <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
     <RestoreLockedMode Condition="$(ContinuousIntegrationBuild) == 'true'">true</RestoreLockedMode>