Fix presubmit-link-check.sh

[adrcunha]

Run markdown_build_tests directly to:

• fix readonly var warning
• skip unnecessary tests

/cc @RichieEscarez

[adrcunha]

#1963 (comment) is an example of bad link detection on presubmit (forced). Note that it doesn't block the submission, even though it is reported in the PR thead (it's not marked "required" in the checks table below).

Since this PR doesn't contain any markdown files, if you click on the details of the markdown-link-check below you'll see that it passed and states so.

/cc @eallred-google

[RichieEscarez]

@adrcunha, the last comment links back to this PR. Im confused by what the "example" is?

[adrcunha]

@adrcunha, the last comment links back to this PR. Im confused by what the "example" is?

Apologies, the passing test removed the comment pointing to the failure. The link to Spyglass is https://prow.knative.dev/view/gcs/knative-prow/pr-logs/pull/knative_docs/1963/pull-knative-docs-markdown-link-check/1191819431852904452

[RichieEscarez]

/lgtm
/approve

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