Keep symlinks in cuDNN installations

[bartoldeman]

Fixes easybuilders/easybuild-easyconfigs#13771

[bartoldeman]

Test report by @bartoldeman

Overview of tested easyconfigs (in order)

• SUCCESS cuDNN-8.0.4.30-CUDA-11.0.2.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
build-node.computecanada.ca - Linux centos linux 7.9.2009, x86_64, Intel Xeon Processor (Skylake, IBRS), Python 3.7.7
See https://gist.github.com/6d09bf5df3a83a470b208e37461ed728 for a full test report.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS cuDNN-7.6.4.38-gcccuda-2019a.eb
• SUCCESS cuDNN-7.6.4.38-gcccuda-2019b.eb
• SUCCESS cuDNN-8.0.4.30-CUDA-11.0.2.eb
• SUCCESS cuDNN-8.0.4.30-CUDA-11.1.1.eb
• SUCCESS cuDNN-8.0.5.39-CUDA-11.1.1.eb

Build succeeded for 5 out of 5 (5 easyconfigs in total)
node3302.joltik.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6242 CPU @ 2.80GHz (cascadelake), Python 3.6.8
See https://gist.github.com/06f0f075289986befc16b88b8b6676b8 for a full test report.

[boegel]

lgtm