Replace the use of actions/checkout with a manual checkout

v3 of this action triggers deprecation warnings and will stop working
soon, while v4 doesn't work under Ubuntu 18.04 which we use for ODBC
testing due to actions/checkout#1442

.github/workflows/ci.yml

@@ -79,7 +79,21 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        run: |
+          case "${{matrix.container}}" in
+            ubuntu:18.04)
+              export DEBIAN_FRONTEND=noninteractive
+
+              apt-get update -qq
+              apt-get install -qq git
+              ;;
+          esac
+
+          git init soci
+          cd soci
+          git remote add origin https://github.com/SOCI/soci.git
+          git fetch --depth=1 origin $GITHUB_SHA
+          git checkout FETCH_HEAD
 
       - name: Set environment variables
         run: |