Allow skipping package build in packages.targets

Introduced a property 'SkipBuildPackages' that allows to skip
creating NuGet packages (similar to ['SkipTests'](https://github.com/dotnet/buildtools/blob/1fe1371d44b58c20117ade7c6b79e258833b2da0/src/Microsoft.DotNet.Build.Tasks/Targets/tests.targets#L24) and ['SkipSigning'](https://github.com/dotnet/buildtools/blob/2dc23399c4ac1b2c6496dd8e600697c666005e29/src/Microsoft.DotNet.Build.Tasks/Targets/sign.targets#L6)).

This helps on Mono/Linux where getting the correct libgit2 version
is a bit cumbersome and I'm not interested in the packages atm.