Use stable version of actions/checkout

Author: hsbt

.github/workflows/macos.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     runs-on: macos-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - name: Install dependencies
       run: |
         gem install bundler --no-document

.github/workflows/ubuntu.yml

@@ -9,7 +9,7 @@ jobs:
       matrix:
         ruby: [ 'jruby-9.2.17.0', 'head', '3.0', '2.7', '2.6', '2.5', '2.4' ]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:

.github/workflows/windows.yml

@@ -9,7 +9,7 @@ jobs:
       matrix:
         ruby: [ 'head', '3.0', '2.7', '2.6', '2.5', '2.4' ]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: