Flutter & Github Actions: Create unsigned APK (Execution failed for task ':app:validateSigningRelease')

Question

I am using GitHub actions to build my Flutter APK for testing purposes. Recently I built and signed my app and released it to Google Play, however, by doing this I broke my GitHub Action.

I get this error now when my action runs:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:validateSigningRelease'.
> Keystore file not set for signing config release

This is the action:

name: APK

on:
  push:
    branches: [master]
  pull_request:
    branches: [master]

jobs:
  build:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v2
      - uses: actions/setup-java@v2
        with:
          distribution: 'zulu'
          java-version: '11'
      - uses: subosito/flutter-action@v1
      - run: flutter pub get
      - run: flutter test
      - run: flutter build apk --no-tree-shake-icons
      - run: flutter build appbundle --no-tree-shake-icons
      - run: sudo chown -R $USER:$GROUP ~
      - uses: actions/upload-artifact@v2
        with:
          name: apk
          path: build/app/outputs/flutter-apk/app-release.apk

So I guess there are two ways to solve this and I'd like to have an answer to both:

1. How can I continue to build unsigned APKs with GitHub Actions?
2. How can I build an actual signed APK ready for release with GitHub Actions?

Keep in mind that building locally works fine (because all required files are present locally to build a signed APK).

---

Edit #01:

My build.gradle looks like this:

...
android {
    compileSdkVersion 31

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

    kotlinOptions {
        jvmTarget = '1.8'
    }

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }

    defaultConfig {
        applicationId "at.test.app"
        minSdkVersion 21
        targetSdkVersion 30
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

    signingConfigs {
        release {
            keyAlias keystoreProperties['keyAlias']
            keyPassword keystoreProperties['keyPassword']
            storeFile keystoreProperties['storeFile'] ? file(keystoreProperties['storeFile']) : null
            storePassword keystoreProperties['storePassword']
        }
    }
    buildTypes {
        release {
            signingConfig signingConfigs.release
        }
    }

}
...