Upgrade actions/checkout to v2 to prevent errors

> Are you using "actions/checkout@v2" or "actions/checkout@v1"? This sounds like a race condition that was fixed in v2.
> actions/checkout#222 (comment)

> Closing since I think this is fixed by "actions/checkout@v2".
> actions/checkout#23 (comment)

.github/workflows/android.yml

@@ -29,7 +29,7 @@ jobs:
           target: ${{ matrix.qt_target }}
           # Architecture for Windows/Android
           arch: ${{ matrix.qt_arch }}  
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 1
       - name: build android

.github/workflows/ios.yml

@@ -25,7 +25,7 @@ jobs:
           version: ${{ matrix.qt_ver }}
           # Target platform for build
           target: ${{ matrix.qt_target }}
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 1
       - name: build ios

.github/workflows/macos.yml

@@ -26,7 +26,7 @@ jobs:
         with:
           version: ${{ matrix.qt_ver }}
           cached: 'false'
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 1
       - name: build macos

.github/workflows/ubuntu.yml

@@ -27,7 +27,7 @@ jobs:
           cached: 'false'
       - name: ubuntu install GL library
         run: sudo apt-get install -y libglew-dev libglfw3-dev
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 1
       - name: build ubuntu

.github/workflows/windows-mingw.yml

@@ -75,7 +75,7 @@ jobs:
           tools: ${{ matrix.qt_tools }}
           aqtversion: '==0.10.0'
           cached: 'false'
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 1
       - name: Qt 5 environment configuration

.github/workflows/windows.yml

@@ -73,7 +73,7 @@ jobs:
           aqtversion: '==0.10.0'
           cached: 'false'
       # 拉取代码
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
         with:
           fetch-depth: 1
       # msvc编译

README.md

@@ -143,7 +143,7 @@ dll，以此来保证在大部分Windows环境都能正常运行。
 
 2. 获取项目代码
 
-这一步用Actions官方核心模板：actions/checkout@v1
+这一步用Actions官方核心模板：actions/checkout@v2
 
 3. 执行qmake、make