Switch Emulator CI to Ubuntu

[ByteHamster]

Description

GitHub switched their MacOS runners to ARM, which makes the Android emulator fail to start. Since we introduced the CI workflow, GitHub upgraded the Ubuntu runners as well, now supporting hardware acceleration. This means we no longer need MacOS. The Ubuntu runner is also about 2 times faster.

Checklist

• I have read the contribution guidelines: https://github.com/AntennaPod/AntennaPod/blob/develop/CONTRIBUTING.md#submit-a-pull-request
• I have performed a self-review of my code
• I have run the automated code checks using ./gradlew checkstyle spotbugsPlayDebug spotbugsDebug :app:lintPlayDebug
• My code follows the style guidelines of the AntennaPod project: https://github.com/AntennaPod/AntennaPod/wiki/Code-style
• I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
• If it is a core feature, I have added automated tests