Update NVIDIA driver symlink script

[casparvl]

We'll need the following variant symlinks to be in place before this script can work as intended:

ln -s '$(EESSI_202506_NVIDIA_OVERRIDE:-/cvmfs/software.eessi.io/defaults/nvidia)' /cvmfs/software.eessi.io/versions/2025.06/compat/linux/x86_64/lib/nvidia
ln -s '$(EESSI_202506_NVIDIA_OVERRIDE:-/cvmfs/software.eessi.io/defaults/nvidia)' /cvmfs/software.eessi.io/versions/2025.06/compat/linux/aarch64/lib/nvidia
ln -s '$(EESSI_202506_NVIDIA_OVERRIDE:-/cvmfs/software.eessi.io/defaults/nvidia)' /cvmfs/software.eessi.io/versions/2025.06/compat/linux/riscv64/lib/nvidia

And then:

ln -s '$(EESSI_NVIDIA_OVERRIDE_DEFAULT:-/dev/null)' /cvmfs/software.eessi.io/defaults/nvidia

This can then be quite easily tested from within the container:

./eessi_container.sh -a rw -r software.eessi.io -b $<host-software-layer-scripts>:/software-layer-scripts --nvidia all
cd /software-layer-scripts/scripts/gpu_support/nvidia
./link_nvidia_host_libraries.sh

This should error out stating that the variant symlink resolves to /dev/null. Then, you can change /etc/cvmfs/default.local to set e.g. EESSI_NVIDIA_OVERRIDE_DEFAULT (e.g. to /opt/eessi/nvidia) and run the linking script again - this should the install the symlinks.

[casparvl]

Although we don't have the symlinks yet, I can actually already test this in the container - it will just create the symlinks in /cvmfs/software.eessi.io/versions/2025.06/compat/linux/x86_64/lib/nvidia/ in the writeable overlay. That's fine.

What I did:

$ cd /software-layer-scripts/scripts/gpu_support/nvidia/
$ umask 0022
$  source /cvmfs/software.eessi.io/versions/2025.06/init/lmod/bash
# For some reason this failed to load the module - some module cache issue?
$ module load EESSI/2025.06
$ cat > dummy.c <<'EOF'
int main(void) { return 0; }
EOF
$ gcc -Wall -Wl,--no-as-needed -lcuda dummy.c -o dummy -L /cvmfs/software.eessi.io/versions/2025.06/compat/linux/x86_64/lib/nvidia/
# singularity has /.singularity.d/libs with the CUDA drivers in the LD_LIBRARY_PATH, but those are not the ones we want to find...
$  unset LD_LIBRARY_PATH
$ ldd dummy
Apptainer> ldd dummy
        linux-vdso.so.1 (0x00007ffc59bb4000)
        libcuda.so.1 => /cvmfs/software.eessi.io/versions/2025.06/compat/linux/x86_64/lib/nvidia/libcuda.so.1 (0x000014f19b377000)
...

Works as intended. After implementing the variant symlinks, we should retest, try to use the EESSI_NVIDIA_OVERRIDE_DEFAULT symlink, and, once that works, try again using the EESSI_202506_NVIDIA_OVERRIDE variant symlink.