Fix appveyor build nuget repository path.

Author: evoskuil

templates/gsl.build.cmd

@@ -147,7 +147,7 @@ exit /b 0
 SETLOCAL ENABLEEXTENSIONS
 SET "parent=%~dp0"
 SET "path_base=%~1"
-SET "nuget_pkg_path=%~1\\..\\nuget"
+SET "nuget_pkg_path=%~1\\.nuget\\packages"
 SET "msbuild_args=/verbosity:minimal /p:Platform=%~2 /p:Configuration=%~3"
 SET "proj_version=%~4"
 SET "msbuild_exe=msbuild"