Provide built-in CHECK_RESULT_URL environment variable

We have checks that ensure we can successfully send email through our service. I include some of the environment details like check run in the body so that I can tie back to the original check and result.

For now I’m creating a link back to the exact result page, which has proved invaluable for troubleshooting:

<a href="https://app.checklyhq.com/accounts/${process.env.ACCOUNT_ID}/checks/${process.env.CHECK_ID}/results/${process.env.CHECK_RESULT_ID}"> View check in Checkly </a>

This is a little brittle though and could be simplified with a simple Checkly-controlled variable CHECK_RESULT_URL