Add holiday messaging for PRs in CI

[BenHenning]

Explanation

A previous attempt to add automatic messaging to PRs didn't work, so manually adding something that will automatically run until the team's multi-person time-off period ends. I'll manually be adding messages to existing PRs since this only operates for new PRs.

Some caveats:

1. I can't test this until the PR is submitted
2. It won't work for first-time contributors since their workflows need to be run by a team member with write access, but everyone else should see the message
3. The workflow will need to be manually disabled once we return (doing a date-based if check is a bit complicated, and I didn't want to have to debug it since this is a temporary thing, anyway)

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

For UI-specific PRs only

N/A -- CI infrastructure PR.

[BenHenning]

Thanks! Enabling auto-merge since the only workflows that could actually fail from this change didn't.

[oppiabot]

Unassigning @seanlip since they have already approved the PR.

[oppiabot]

Hi @BenHenning, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!