Fix Build Failure Due to Nsync Not Being Found Correctly

[jordanozang]

Description

When fetching nsync via FetchContent_Declare, force it to be downloaded instead of attempting to find an installed CMake package first.

Motivation and Context

While nsync 1.26 and lower did not export CMake targets, versions from 1.27 onwards do. Also, some package maintainers chose to create CMake targets as part of their distribution for previous versions. If these are installed on the user's system, onnxruntime will currently fail to build. This is because onnxruntime is incompatible with these targets and instead expects the library to be provided as a subdirectory in the _deps folder. Preventing cmake from searching system paths for the targets solves this build failure.

This is the error I was getting:

CMake Error at onnxruntime.cmake:225 (target_link_libraries):
  Target "onnxruntime" links to:

    nsync::nsync_cpp

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

I think the planned solution had been #20413, but it looks like it was too difficult to remove nsync without affecting existing functionality.

• The error messages here are not exactly the same, but I suspect the root issue is the same: [Build] add_library cannot create ALIAS target "nsync::nsync_cpp" because target "nsync_cpp" does not already exist #20031

[snnn]

I will remove this dependency by the next release (1.19.0)