Report E2E test failures to Slack again

[dechov]

Changes proposed in this Pull Request

Give E2E testing workflow access to environment variables necessary for reporting failures to Slack (with screenshots).

cc @kalessil (particularly in case omitting these variables was intentional)

Testing instructions

Added a temporary commit (7f7dde2) to trigger a failure and verify report in Slack – will reset/revert before merge.

---

• Added changelog entry (or does not apply)
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

[dechov]

Doesn't look like it was reported 🤔

Edit: ah, "channel not found" error. Not sure what the current E2E_SLACK_CHANNEL value is, but if it's the name, it might not work, since it looks like it needs to be the ID [source, doc]. Will change it and retry. Still doesn't work.

[dechov]

Ah, it works now: it uses the E2E_CHANNEL_NAME variable after all – I need not have doubted the README 😄

I've committed that one to the workflow and added it to the repository secrets in GH, but there are some issues with the reporting that we should solve before going forward with this PR: much of the output is blank because the Automattic/puppeteer-utils library assumes Travis is being used.

[dechov]

Should likely switch to using Slack reporting from WC core E2E utils instead. Discussion in p1625275687050700-slack-C026J0AL8NS