fix: shorebird releases get-apks generates empty folder

[bryanoltman]

App ID: e5868886-9bec-43a6-87e3-8700059674e7

Description

Running shorebird releases get-apks with no arguments outputs the following:

⑆ shorebird releases get-apks 
✓ Fetching releases (0.2s)
Which release would you like to generate an apk for? 1.0.0+7
✓ Fetching aab artifact (0.2s)
✓ Downloading aab (100%) (0.4s)
✓ Building apks for release 1.0.0+7 (app: e5868886-9bec-43a6-87e3-8700059674e7) (2.4s)
apk(s) generated at /Users/bryanoltman/Documents/sandbox/macos_signing/build/app/outputs/shorebird-apk

But /Users/bryanoltman/Documents/sandbox/macos_signing/build/app/outputs/shorebird-apk is just an empty directory. Using the --no-universal outputs the expected APKs

Steps To Reproduce

1. Create an Android release
2. Run shorebird releases get-apks
3. See that the shorebird-apk directory is empty

Expected Behavior

A universal APK is created (or the command fails because it can't do what the user is asking for)

[NavidShokoufeh]

same here!