Avoid link-checking IANA links

[omus]

IANA servers can be flaky so it's probably best we avoid checking these links:

[ Info: CheckDocument: running document checks.
┌ Error: `curl -sI --proto =http,https,ftp,ftps --user-agent 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36' -H 'accept-encoding: gzip, deflate, br' https://data.iana.org/time-zones/data/tzfile.5.txt --max-time 10 -o /dev/null --write-out '%{http_code} %{url_effective} %{redirect_url}'` failed:
│   exception =
│    failed process: Process(`curl -sI --proto =http,https,ftp,ftps --user-agent 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36' -H 'accept-encoding: gzip, deflate, br' https://data.iana.org/time-zones/data/tzfile.5.txt --max-time 10 -o /dev/null --write-out '%{http_code} %{url_effective} %{redirect_url}'`, ProcessExited(28)) [28]
│    
└ @ Documenter.DocChecks ~/.julia/packages/Documenter/bYYzK/src/Utilities/Utilities.jl:32
┌ Error: `curl -sI --proto =http,https,ftp,ftps --user-agent 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36' -H 'accept-encoding: gzip, deflate, br' https://data.iana.org/time-zones/data/tzfile.5.txt --max-time 10 -o /dev/null --write-out '%{http_code} %{url_effective} %{redirect_url}'` failed:
│   exception =
│    failed process: Process(`curl -sI --proto =http,https,ftp,ftps --user-agent 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36' -H 'accept-encoding: gzip, deflate, br' https://data.iana.org/time-zones/data/tzfile.5.txt --max-time 10 -o /dev/null --write-out '%{http_code} %{url_effective} %{redirect_url}'`, ProcessExited(28)) [28]
│    
└ @ Documenter.DocChecks ~/.julia/packages/Documenter/bYYzK/src/Utilities/Utilities.jl:32
[ Info: Populate: populating indices.
ERROR: LoadError: `makedocs` encountered an error (:linkcheck). Terminating build before rendering.

– https://github.com/JuliaTime/TimeZones.jl/actions/runs/5946262840/job/16126554657#step:7:35