Add library dir to singularity config

[chris-cheshire]

---

name: New Config
about: A new cluster config

Please follow these steps before submitting your PR:

• If your PR is a work in progress, include [WIP] in its title
• Your PR targets the master branch
• You've included links to relevant issues, if any

Steps for adding a new config profile:

• Add your custom config file to the conf/ directory
• Add your documentation file to the docs/ directory
• Add your custom profile to the nfcore_custom.config file in the top-level directory
• Add your custom profile to the README.md file in the top-level directory
• Add your profile name to the profile: scope in .github/workflows/main.yml

[ChristopherBarrington]

Checked using fetchngs 1.10.0 and looks good.

Copied the added path to a custom.config. Running the pipeline with the custom config used the cached images whereas without the custom config the images were downloaded to my own cache.