updating yml

.gitlab-ci.yml

-image: dart:stable
+image: cirrusci/flutter:stable
 
 stages:
   - build
   - deploy
 
-variables:
-  FLUTTER_VERSION: stable
-
-before_script:
-  - apt-get update && apt-get install -y curl unzip
-  - curl -O https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_${FLUTTER_VERSION}.zip
-  - unzip -o flutter_linux_${FLUTTER_VERSION}.zip
-  - export PATH="$PATH:$PWD/flutter/bin"
-  - flutter doctor
-
 build:
   stage: build
   script: