fix build.sh on XDG_DATA_HOME and copy nuget.exe

Related to #193. The whole explanation is there: https://github.com/aspnet/KRuntime/pull/193#issuecomment-43680415

The correct way to verify the variable XDG_DATA_HOME is using -z.
To copy a file on bash you need to specify the filename also, not the
folder only.
Added execute permission to build.sh.