Disable signing in NuGet.Client rather than providing our own key in multiple parameters

[dagood]

source-build/repos/nuget-client.proj

Lines 19 to 20 in 4ca4d03

	<EnvironmentVariables Include="MS_PFX_PATH=$(NuGetKeyFilePath)" />
	<EnvironmentVariables Include="NUGET_PFX_PATH=$(NuGetKeyFilePath)" />

#646 (comment)

[@crummel] Yeah, there's now two different variables used for the PFX path, we will attempt to delay-sign and fail without this. I can look at whether we can disable signing entirely, that seems like the better option.