Remove schedule from build-main workflow

[gtdiehl]

Given the low number of changes to the repository I feel we can get away with running Workflows with PR merges and commits to main

Pull request recommendations:

• [ ] Name your pull request your-development-type/short-description. Ex: feature/read-tiff-files
• [ ] Link to any relevant issue in the PR description. Ex: Resolves [gh-12], adds tiff file format support
• [ ] Provide context of changes.
• [ ] Provide relevant tests for your feature or bug fix.
• [ ] Provide or update documentation for any feature added by your pull request.

Thanks for contributing!