Update regression-test workflow to only run on workflow dispatch

[Sarahligbe]

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Description

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Related Issue (Link to issue ticket)

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Motivation and Context

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How Has This Been Tested?

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Screenshots (if appropriate - Postman, etc):

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
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Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.