Modify .nuget/NuGet.targets to fix problem with nuget downloads under mo...

...no and osx

Exec cmd (under xbuild@osx):
EnableNuGetPackageRestore=ture Runtime=MONO xbuild ClojureCLR.sln

Error message:
clojure-clr/Clojure/ClojureCLR.sln (default targets) ->
(Build target) ->
clojure-clr/Clojure/Clojure/Clojure.csproj (default targets) ->
clojure-clr/Clojure/.nuget/nuget.targets (RestorePackages target) ->

	clojure-clr/Clojure/.nuget/nuget.targets: error : Command 'mono --runtime=v4.0.30319 clojure-clr/Clojure/.nuget/NuGet.exe install "packages.config" -source ""  -RequireConsent -o "clojure-clr/Clojure/packages"' exited with code: 1.

Reason:
different of position PakcageConfig.