Fixing if syntax in periodic push workflow

[jerinphilip]

When you use expressions in an if conditional, you may omit the expression syntax (${{ }}) because GitHub automatically evaluates the if conditional as an expression, unless the expression contains any operators. If the expression contains any operators, the expression must be contained within ${{ }} to explicitly mark it for evaluation. For more information about if conditionals, see "Workflow syntax for GitHub Actions."

https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions#about-contexts-and-expressions

Copying over from https://github.com/jerinphilip/gh-workflow-multi-repo-sync-mirror/blob/3ccc0f66ba97e9a3662ade8f9a5e1c421529df0a/.github/workflows/mirror.yml#L23, should work hopefully.