fix(ci): skip Mattermost notifications for external pull requests

[outdooracorn]

The CI job that sends a Mattermost message when a PR is (re-)opened, defined in .github/workflows/mattermost.yml, fails with Error: Input required and not supplied: MATTERMOST_WEBHOOK_URL for PRs from external repositories.

Contributors have expressed a desire for CI workflows to not fail for reasons outside of their control (1 2).

This would involve either:

• Modifying the failing job so that it successfully runs for all PRs targeted at this repo (as done in 1)
• Preventing the job from running for PRs from external repositories (this PR)

The later is the safer option and doesn't risk diverting the team's attention away from the focus topic, so I'm inclined to go that route to start with.

[addshore]

LGTM!
As a side note, it would be incredibly helpful to establish a reliable way for open-source contributors to get PRs reviewed so they don't stall out. If chat and GitHub notifications aren't working or risk distracting the core team, what process would work best for you?
I’d love to contribute to the wbstack repos more often, but the historical friction in getting even small changes merged has been a major blocker.