Use NuGet "snupkg" format for symbols package

https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages-snupkg

Symbol package publishing is only supported by the NuGet V3 API.

NuGet.org supports its own symbols server repository and only accepts the
new symbol package format - .snupkg.

cake-build/cake#2362
cake-build/cake#3331

HOWEVER: This is only supported by the upcoming Cake 1.2 release.  This PR ends up being about
turning off the IncludeSymbols=true by default,
requiring individual .csproj files to define it instead.

Workaround for the 500 exception we are getting
from the NuGet server on our public packages.

Author: Thomas Harold

build-net5.cake

@@ -328,7 +328,9 @@ Task("Package")
                     MSBuildSettings = new DotNetCoreMSBuildSettings().SetVersion(packageVersion),
                     NoBuild = true,
                     OutputDirectory = artifactsDir,
-                    IncludeSymbols = true
+                    //TODO: Put this back in once Cake 1.2 is released, which adds support for "SymbolPackageFormat"
+                    //IncludeSymbols = true,
+                    //SymbolPackageFormat = "snupkg", // https://github.com/cake-build/cake/pull/3331
                 });
             }
         }