CI: sync submodules before running actions/checkout

This action runs "git submodule foreach" early which will fail if a
submodule has been deleted but is still present in the repo (see
actions/checkout#354).

.github/workflows/ci.yaml

@@ -52,6 +52,9 @@ jobs:
       - name: Set JDK 16 as default
         run: echo "/usr/lib/jvm/java-16-openjdk-amd64/bin" >> $GITHUB_PATH
 
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -97,6 +100,9 @@ jobs:
       - name: Set JDK 16 as default
         run: echo "/usr/lib/jvm/java-16-openjdk-amd64/bin" >> $GITHUB_PATH
 
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -190,6 +196,9 @@ jobs:
          )"
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -230,6 +239,9 @@ jobs:
          )"
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -270,6 +282,9 @@ jobs:
          )"
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -310,6 +325,9 @@ jobs:
          )"
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -358,6 +376,9 @@ jobs:
       - name: Set JDK 8 as default
         run: echo "/usr/lib/jvm/java-8-openjdk-amd64/bin" >> $GITHUB_PATH
 
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -394,6 +415,9 @@ jobs:
       SONATYPE_USER: ${{ secrets.SONATYPE_USER_ORGSCALALANG }}
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -428,6 +452,9 @@ jobs:
                                            # Make sure you have the write permissions to the repo: https://github.com/lampepfl/dotty-website
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -474,6 +501,9 @@ jobs:
       SONATYPE_USER: ${{ secrets.SONATYPE_USER_ORGSCALALANG }}
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -556,6 +586,9 @@ jobs:
                                            # Make sure you have the write permissions to the repo: https://github.com/lampepfl/dotty-website
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2
 
@@ -601,6 +634,9 @@ jobs:
       SONATYPE_USER: ${{ secrets.SONATYPE_USER }}
 
     steps:
+      - name: Synchronize submodules
+        run: git submodule sync || true
+
       - name: Checkout cleanup script
         uses: actions/checkout@v2