GitHub Actions workflow to build Flutter app and create Release, put this file under `.github/workflows` folder.

build-and-release.yml

on:
  push:
    branches:
      - master

name: Build and Release Apps
jobs:
  build:
    name: Build Apps
    runs-on: macos-latest
    steps:
      - name: Export Release Timestamp
        run: echo "::set-env name=APP_VERSION::release_$(date +'%Y-%m-%d_%H-%m-%S')"
      - name: Checkout repository
        uses: actions/checkout@v1
      - name: Set up Java
        uses: actions/setup-java@v1
        with:
          java-version: "12.x"
      - name: Set up Flutter
        uses: subosito/flutter-action@v1
        with:
          channel: "stable"
      - name: Install pub Dependencies
        run: flutter pub get
      - name: Run Tests
        run: flutter test
      - name: Build Android App
        run: flutter build apk --split-per-abi
      - name: Build iOS App
        run: |
          flutter build ios --no-codesign
          cd build/ios/iphoneos
          mkdir Payload
          cd Payload
          ln -s ../Runner.app
          cd ..
          zip -r app.ipa Payload
      - name: Release Apps
        uses: ncipollo/release-action@v1
        with:
          tag: ${{ env.APP_VERSION }}
          name: ${{ env.APP_VERSION }}
          token: ${{ secrets.GITHUB_TOKEN }}
          artifacts: "build/app/outputs/apk/release/*.apk,build/ios/iphoneos/app.ipa"

set-env is deprecated in github actions and thus Export Release Timestamp is giving a error.
You can make changes according to the new environment file handling on github
Please update the run command with the following new syntax to fix it.

echo "APP_VERSION=release_$(date +'%Y-%m-%d_%H-%m-%S')" >> $GITHUB_ENV