[request] replace custom macos cert imports in github action example with Apple-Actions/import-codesign-certs

[FabianLars]

something's up with our script as it results in timeouts during signing/notarization for many, see https://discord.com/channels/616186924390023171/1309029734105092196 & tauri-apps/tauri-action#941

https://github.com/Apple-Actions/import-codesign-certs seems to be widely used and also talked about in the issue above so i think it makes sense to use that instead of the custom stuff. this would also shrink down the example a bit