Don't run xrefcheck on the gnu.org link in values.md

[winterqt]

This check frequently fails in CI, due to gnu.org not liking the hosted GHA runners' network, or something.

[winterqt]

This failure is expected though, because the NixCon link is broken at the moment.

[infinisil]

Nice!

[winterqt]

untrusted-pr/doc/resources.md:40:3-53: bad reference:
  The reference to "Open collective" failed verification.
  Response timeout
  when processing an external link:
    https://opencollective.com/nixos

please lmao

[winterqt]

Re-ran it, works fine now... perhaps we should add some retries (3? 5?) to the xrefcheck invocation?

[infinisil]

Can be a follow-up if necessary :)