Update smoke-test.yml to only alert on scheduled runs

[OliviaLynn]

Problem & Solution Description (including issue #)

Updating the smoke test's "alert slack" step to only trigger on scheduled runs

This lets us manually trigger smoke tests without cluttering the slack

Code Quality

• [x] My code follows the code style of this project

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6194876) 100.00% compared to head (0b659e4) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #114 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 1 1 ========================================= Hits 1 1 ```

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