For iOS apps, you need a Mac. Maybe (and this is theoretical) you could set up a combined Fastlane & CI integration that builds the apps using some cloud service, such as Macincloud, Bitrise, etc. Fastlane would then build and upload the iOS test builds automatically so can use them on your phone using the internal testing group in TestFlight. When everything seems fine, then you can just promote the internal test build to production. Since Flutter custom renders everything, you could do most of the development on a Linux machine and test on Android emulator/device.

For just Android, you don't need a Mac at all.