ci: Exclude explorer.optimism.io from link checker.

[ajsutton]

Description

It regularly rejects requests from CI due to DoS prevention even though the link is valid.

[changeset-bot]

⚠️ No Changeset found

Latest commit: b1c0f8f

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[netlify]

✅ Deploy Preview for opstack-docs canceled.

Name	Link
🔨 Latest commit	b1c0f8f
🔍 Latest deploy log	https://app.netlify.com/sites/opstack-docs/deploys/64583ec15dfebf0008ef0d44

[refcell]

LGTM

[mergify]

This PR has been added to the merge queue, and will be merged soon.

[mergify]

This PR is next in line to be merged, and will be merged as soon as checks pass.