Fix CI Caching by cloning first

The cache action requires the checksum of the packages.lock.json files,
but can't find them because the repository wasn't cloned yet. So clone
the repository first

.github/workflows/main.yml

@@ -1,13 +1,17 @@
 name: CI
 
-on: [ "push", "pull_request" ]
+on: ["push", "pull_request"]
 
 jobs:
   build:
     runs-on: ubuntu-20.04
 
     steps:
-      - uses: actions/cache@v2
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+
+      - uses: actions/cache@v3
         with:
           path: |
             ~/.nuget/packages
@@ -16,10 +20,6 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-nuget-
 
-      - uses: actions/checkout@v2
-        with:
-          submodules: true
-
       - name: Setup .NET
         uses: actions/setup-dotnet@v1
         with: