Potential fix for code scanning alert no. 1: Workflow does not contain permissions

[donatj]

Potential fix for https://github.com/donatj/Circle-Generator/security/code-scanning/1

To fix the problem, add a permissions block to the workflow to explicitly set the minimum required permissions for the GITHUB_TOKEN. Since the workflow only checks out code and builds it, it only needs read access to repository contents. The best way to do this is to add permissions: contents: read at the top level of the workflow (just after the name field and before on). This ensures all jobs in the workflow inherit these minimal permissions unless overridden.

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