Use TARGET_DIRECTORY for Kron.cpp CUDA property

[ultimatile]

Building src/linalg/Kron.cpp for GPU should be handled by nvcc (or another CUDA-family compiler).
However, under the current configuration, the LANGUAGE CUDA property does not propagate to the top-level CMake configuration, causing the file to be compiled incorrectly with a C++ compiler such as g++.
This PR corrects the issue by using TARGET_DIRECTORY, enabling Kron.cpp to be built properly with nvcc.
This fix may also resolve the ordering issue mentioned in the comment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.19%. Comparing base (eb3e3be) to head (0efc3ab).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #710   +/-   ##
=======================================
  Coverage   30.19%   30.19%           
=======================================
  Files         215      215           
  Lines       38335    38335           
  Branches    15474    15474           
=======================================
  Hits        11576    11576           
  Misses      24858    24858           
  Partials     1901     1901           

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[IvanaGyro]

I wonder why CMAKE_SOURCE_DIR doesn't cover the file.

[ultimatile]

Sorry for the late response. I needed some time to investigate why DIRECTORY ${CMAKE_SOURCE_DIR} does not work.

Both DIRECTORY ${CMAKE_SOURCE_DIR} and TARGET_DIRECTORY cytnx work correctly when Cytnx is built as a standalone project.
The difference appears when Cytnx is used as a subdirectory.
In that case, ${CMAKE_SOURCE_DIR} no longer refers to Cytnx's root, but to the parent project's root instead.
Therefore, DIRECTORY ${CMAKE_SOURCE_DIR} points to an incorrect path and does not work.

DIRECTORY ${PROJECT_SOURCE_DIR} also worked.