GetPackageDescription task caches description table without considering timestamps

GetPackageDescription task caches the description table on the current MSBuild node using the full path to descriptions.json as the key. So if I'm adding a new package, do a full build of dotnet/runtime for example, get the failure that I need to add a description to the package. I go add it, then do a full build again, it will keep failing until I kill MSBuild nodes and try again.

https://github.com/dotnet/arcade/blob/8078d8f3f77b7e8b7f6e289cf82cfdfa9c7a9355/src/Microsoft.DotNet.Build.Tasks.Packaging/src/GetPackageDescription.cs#L16-L17

https://github.com/dotnet/arcade/blob/8078d8f3f77b7e8b7f6e289cf82cfdfa9c7a9355/src/Microsoft.DotNet.Build.Tasks.Packaging/src/GetPackageDescription.cs#L64-L70

Similar to: https://github.com/dotnet/arcade/commit/8820dcbdf6c93291b161a255c04f60b6b906c291#diff-38acd30f6f6e533a1d7b16bdb0ac3809

cc: @ericstj @Anipik @ViktorHofer

FYI: @bartonjs who reached out as he was hitting this.