CI: use fixed versions to avoid builds suddenly failing.

Author: greenrobot-team

.github/workflows/dart.yml

@@ -16,7 +16,7 @@ jobs:
   generator:
     runs-on: ubuntu-20.04
     container:
-      image: google/dart:latest
+      image: google/dart:2.17.0
     steps:
       - uses: actions/checkout@v2
       - name: Install ObjectBox C-API
@@ -30,6 +30,9 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: 3.0.0
+          cache: true
       - run: ./tool/init.sh
 
   lib:
@@ -41,7 +44,7 @@ jobs:
           - macos-11
           - ubuntu-20.04
         sdk:
-          - stable
+          - 2.17.0
           - 2.16.2
           - 2.15.1
           - 2.14.4
@@ -67,6 +70,8 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - uses: dart-lang/setup-dart@v1
+        with:
+          sdk: 2.17.0
       - uses: actions/checkout@v2
       - name: Install ObjectBox C-API
         run: ./install.sh
@@ -90,15 +95,16 @@ jobs:
           - windows-2022
           - macos-11
           - ubuntu-20.04
-        channel:
-          - beta
-          - stable
+        flutter-version:
+          - 3.0.0
+          - 2.0.0 # Lowest supported version, based on lowest supported Dart SDK, see pubspec.yaml files
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - uses: subosito/flutter-action@v2
         with:
-          channel: ${{ matrix.channel }}
+          flutter-version: ${{ matrix.flutter-version }}
+          cache: true
       - uses: actions/setup-java@v2 # macos-10.15 and windows-2019 default to Java 8, but Android Plugin requires 11
         with:
           distribution: 'temurin'

.github/workflows/lint.yml

@@ -12,7 +12,7 @@ jobs:
   analyze:
     runs-on: ubuntu-20.04
     container:
-      image: google/dart:latest
+      image: google/dart:2.17.0
     steps:
       - uses: actions/checkout@v2
       - run: dart run build_runner build
@@ -22,7 +22,7 @@ jobs:
       - run: dart format --set-exit-if-changed --fix .
 
   pana:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: axel-op/dart-package-analyzer@v3
@@ -41,10 +41,13 @@ jobs:
           fi
 
   coverage:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: subosito/flutter-action@v2
+        with:
+          flutter-version: 3.0.0
+          cache: true
       - name: Generage test coverage
         working-directory: objectbox
         run: |