Avoid overriding NuGetPackageVersion from environment variable (#122)

We need to allow `NuGetPackageVersion` to be provided as an environment
variable.
Currenlty, the value is unconditionally reassigned to values provided 
in `service_fabric_common.props`.

properties/service_fabric_nuget.props

@@ -4,7 +4,7 @@
 
   <!-- Set Versions. These are used for generating Nuget packages. -->
   <PropertyGroup>
-    <NuGetPackageVersion>$(MajorVersion).$(MinorVersion).$(BuildVersion)</NuGetPackageVersion>
+    <NuGetPackageVersion Condition="'$(NuGetPackageVersion)' == ''">$(MajorVersion).$(MinorVersion).$(BuildVersion)</NuGetPackageVersion>
   </PropertyGroup>
 
   <PropertyGroup>