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If those instances are only for tests/validation it could simplify some overall pipelines quite a bit.

We have a pool of MacOS hardware to run tests, and another pool to perform signing. The latter is a security risk so very tightly controlled and locked down. They are a pita to maintain.

Being able to sign on Linux will allow us to re-use the existing signing infrastructure we use for literally every other platform other than MacOS. It'll be more secure and much less maintenance.

You could use this in addition to regular macOS machines. You can run tests from un-trusted branches, without worrying that something will expose the signing keys.

Then after merging, you have a second machine / set of machines that only does signing. It increases the difficulty of an accidental or intentional leakage of your sensitive keys.

(Note: I have not tested this yet)

That doesn't justify the build part.