Build: Fix finding nvcc (if not in $PATH) with older versions of CMake

[maxhgerlach]

In #3444 we've added a mechanism that helps CMake find nvcc when it's missing in $PATH, but the CUDA toolkit can still be found at a typical location. It has now been observed that this does not work as intended on older versions of CMake like 3.13 or 3.16 (#3545).

CMake would go into an infinite loop with messages like

  You have changed variables that require your cache to be deleted.
  Configure will be re-run and you may have to reset some variables.
  The following variables have changed:
  CMAKE_CUDA_COMPILER= NOTFOUND

NOTFOUND is the initial value assigned to CMAKE_CUDA_COMPILER by check_language, which is written to CMake's build cache then. We override it to a proper value set(CMAKE_CUDA_COMPILER "${CUDAToolkit_BIN_DIR}/nvcc"), but this change only appears to be effective with more recent versions of cmake (I was using 3.23).

In my tests with CMake 3.13 and 3.16 the problem is resolved by clearing the CMAKE_CUDA_COMPILER variable from the cache before setting it to a new value.

Similar infinite loops have been reported earlier (https://public.kitware.com/Bug/view.php?id=14841) and it has been discouraged to override CMAKE_<LANG>_COMPILER inside CMakeLists.txt, but I'd still prefer to contain this logic inside the CMake script.

Fixes #3545

[github-actions]

Unit Test Results

  1 125 files  +     76    1 125 suites  +76   11h 11m 2s ⏱️ + 4m 6s
     813 tests ±       0       755 ✔️ ±    0       58 💤 ±    0  0 ❌ ±0 
22 228 runs  +1 636  15 484 ✔️ +948  6 744 💤 +688  0 ❌ ±0 

Results for commit 48c741e. ± Comparison against base commit 1fbdab5.

♻️ This comment has been updated with latest results.

[github-actions]

Unit Test Results (with flaky tests)

  1 303 files  +   140    1 303 suites  +140   11h 49m 43s ⏱️ + 13m 5s
     813 tests ±       0       755 ✔️ ±       0       58 💤 ±    0  0 ❌ ±0 
25 400 runs  +2 354  17 384 ✔️ +1 392  8 016 💤 +962  0 ❌ ±0 

Results for commit 48c741e. ± Comparison against base commit 1fbdab5.

♻️ This comment has been updated with latest results.

[EnricoMi]

LGTM!