CircleCI Slack orb fails from forks, causing workflows to error out

[cfm]

Description

After #6739, when CircleCI runs on a branch from a fork, the new Slack orb errors out without $SLACK_ACCESS_TOKEN.

Steps to Reproduce

1. Fork freedomofpress/securedrop.
2. Clone your fork and check out a new branch.
3. Commit something on your branch, even if it's just git commit --allow-empty.
4. Push your branch to your fork.
5. Open a pull request from your branch into freedomofpress/securedrop.

Expected Behavior

CircleCI runs and passes , and the Slack orb posts a notification to #securedrop-notifications.

Actual Behavior

In each CircleCI job, step Slack - Sending Notification errors out and the job fails:

In order to use the Slack Orb (v4 +), an OAuth token must be present via the SLACK_ACCESS_TOKEN environment variable.
Follow the setup guide available in the wiki: https://github.com/CircleCI-Public/slack-orb/wiki/Setup

Exited with code exit status 1

CircleCI received exit code 1

• Example from a non-maintainer contributor: https://app.circleci.com/pipelines/github/freedomofpress/securedrop/5633/workflows/5e6342a8-914e-4f7b-ad5a-e7e1476b587b/jobs/68126
• Example from a maintainer (yours truly): https://app.circleci.com/pipelines/github/freedomofpress/securedrop/5637/workflows/e9c890de-b5d8-4672-9608-921dc58f2237/jobs/68139

Comments

#6739 comes from freedomofpress/infrastructure#3225, but we should probably hold off on trying to fix this until we've decided freedomofpress/securedrop-engineering#24.

[zenmonkeykstop]

Do we want notifications from forks? That could get noisy.

[cfm]

Ah, good point, @zenmonkeykstop. I've updated the "Expected Behavior" section: the goal is that CircleCI *passes*, whether or not there's a notification to Slack. CircleCI support confirmed this diagnosis for @nabla-c0d3 in <#6764 (comment)>.