Linkcheck fails on nixos.wiki (403 in CI)

[Samk1710]

Documentation build fails intermittently in GitHub Actions(works fine in browsers) with:

Warning, treated as error:
/home/runner/work/vulnerablecode/vulnerablecode/docs/source/installation.rst:237:broken link: https://nixos.wiki/wiki/Flakes (403 Client Error: Forbidden for url: https://nixos.wiki/wiki/Flakes)
(    installation: line  237) 
Error: Process completed with exit code 2.

Due to this, PR builds are often failing:

Examples:

• https://github.com/aboutcode-org/vulnerablecode/actions/runs/21865658177/job/63106069927?pr=2101
• https://github.com/aboutcode-org/vulnerablecode/actions/runs/21820414546/job/62959598077?pr=2104