How to build apk without signing in Github workflow?

I want to run Github CI and build apk without signing it.

Tried to specify debug buildType, set signingConfig to null and signingConfigs.debug, but it didn't help. Here is my build.gradle:

signingConfigs {
    release {
        keyAlias keystoreProperties['keyAlias']
        keyPassword keystoreProperties['keyPassword']
        storeFile file(keystoreProperties['storeFile'])
        storePassword keystoreProperties['storePassword']
    }
}

buildTypes {
    release {           
        signingConfig signingConfigs.release
    }
}

Another point is that I'm building a flutter application, so this command is used in CI:

- run: flutter build apk --release

How can I build an apk without sign?