[CI] Don't check canonical links in production as part of sanity testing

[chalin]

During production builds under Netlify, we do sanity checks, which include link checking. Our link checker is htmltest, which is run on the generated site files before they are published to the production server. So it doesn't make sense to check canonical links, which point to the production server, since the new site hasn't been deployed yet.

Here's an excerpt from a build log, https://app.netlify.com/teams/opentelemetry/builds/66ad42e51e6650000862745d:

...
4:37:53 PM: tmp/bin/htmltest --log-level 1
4:37:53 PM: htmltest started at 08:37:53 on public
4:37:53 PM: ========================================================================
4:37:53 PM: running in concurrent mode, this is experimental
4:37:55 PM: blog/2023/otel-in-focus-break/index.html
4:37:55 PM:   hitting --- blog/2023/otel-in-focus-break/index.html --> https://opentelemetry.io/blog/2023/otel-in-focus-break/
4:37:55 PM: blog/2023/otel-in-focus-01/index.html
4:37:55 PM:   hitting --- blog/2023/otel-in-focus-01/index.html --> https://opentelemetry.io/blog/2023/otel-in-focus-01/
4:37:56 PM: blog/2023/integrations/index.html
4:37:56 PM:   hitting --- blog/2023/integrations/index.html --> https://opentelemetry.io/blog/2023/integrations/
4:37:56 PM: blog/2023/otel-in-focus-07/index.html
4:37:56 PM:   hitting --- blog/2023/otel-in-focus-07/index.html --> https://opentelemetry.io/blog/2023/otel-in-focus-07/
4:37:56 PM: blog/2023/gc-elections/index.html
4:37:56 PM:   hitting --- blog/2023/gc-elections/index.html --> https://opentelemetry.io/blog/2023/gc-elections/
4:37:56 PM: blog/2022/welcome/index.html
4:37:56 PM:   hitting --- blog/2022/welcome/index.html --> https://opentelemetry.io/blog/2022/welcome/
...

The link checker is "hitting" canonical links, e.g.:

$ curl -s https://opentelemetry.io/blog/2023/otel-in-focus-break | grep 'https://opentelemetry.io/blog/2023/otel-in-focus-break'
... <link rel=canonical href=https://opentelemetry.io/blog/2023/otel-in-focus-break/>...

There is little benefit in checking those canonical links, and in fact, any reported errors would likely be false positives because the new site hasn't been deployed.

I could modify the OTel partial to add a special tag to the canonical link element, but it'll be simpler to just ignore <link> elements with rel attributes. Yes, that'll rule out link elements other than rel="canonical", but this is only meant as a sanity test, and we won't miss out on very many extra ` elements.