GitHub Actions: actions/checkout@master -> v1.1.0 to avoid unexpected failures

See https://github.com/actions/checkout/issues/84
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 04dc708..6ad927f 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -30,7 +30,7 @@
     name: 'Windows (JDK 11)'
     runs-on: windows-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1.1.0
       with:
         fetch-depth: 50
     - name: 'Set up JDK 11'
@@ -50,7 +50,7 @@
       uses: actions/setup-java@v1
       with:
         java-version: 8
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1.1.0
       with:
         fetch-depth: 50
     - name: 'Install Avatica to Maven Local'
@@ -66,7 +66,7 @@
     name: 'macOS (JDK 13)'
     runs-on: macos-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1.1.0
       with:
         fetch-depth: 50
     - name: 'Set up JDK 13'