Automatically update conda lock files.

[joaander]

Description

Automatically update conda lock files on the first day of the month.

Motivation and Context

Dependabot does not know how to update conda environments or lock files. Use a custom GitHub action that runs update-lockfiles.sh and creates a new pull request automatically.

How Has This Been Tested?

I have tested this reusable action with fresnel: glotzerlab/fresnel#240

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.