update-repo job fails (SSL peer certificate or SSH remote key was not OK)

The update-repo CI job currently fails with the following error message (example):

$ nix-channel --update
warning: unable to download 'https://nixos.org/channels/nixpkgs-unstable': SSL peer certificate or SSH remote key was not OK (60); retrying in 270 ms
warning: unable to download 'https://nixos.org/channels/nixpkgs-unstable': SSL peer certificate or SSH remote key was not OK (60); retrying in 549 ms
warning: unable to download 'https://nixos.org/channels/nixpkgs-unstable': SSL peer certificate or SSH remote key was not OK (60); retrying in 1298 ms
warning: unable to download 'https://nixos.org/channels/nixpkgs-unstable': SSL peer certificate or SSH remote key was not OK (60); retrying in 2508 ms
error: unable to download 'https://nixos.org/channels/nixpkgs-unstable': SSL peer certificate or SSH remote key was not OK (60)

I tried restarting the job a couple of times but experienced the same issue each time.

Note that this job only runs on the master branch's CI. As far as I can tell, the CI for MRs is unaffected.