Fix build process for F-Droid

Flutter expects a certain file name when building.
The build will be marked as failed otherwise, even if it completed successfully.
See https://github.com/flutter/flutter/issues/24106.
2 jobs for !17 with fix-fdroid-build in 0 seconds (queued for 83 minutes and 42 seconds)
latest detached
Status Job ID Name Coverage
  Check
failed #1218
docker
analyze

failed #1217
docker
format

 
Name Stage Failure
failed
analyze Check There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace
failed
format Check There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace