print warning if actual triton cache dir is on NFS, not just for default (#6487)
move the logic that prints a warning when triton cache dir is on NFS to
act on the actual calculated cache_dir rather than on the default.
this means that:
- when the default directory (in the user's home directory) is on NFS
but `TRITON_CACHE_DIR` is set to a non-NFS directory, no warning will be
printed whereas prior to this change a spurious and confusing warning
was printed
- when the user's home directory is not on NFS but `TRITON_CACHE_DIR` is
set to an NFS directory, a warning will be printed whereas prior to this
change no warning would be printed
fixes #6486