Fix checkout depth, use action for Kani repo

Author: tautschnig

.github/workflows/update-subtree.yml

@@ -16,15 +16,18 @@ jobs:
     steps:
     - name: Checkout Repository
       uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+
+    - name: Checkout Kani
+      uses: actions/checkout@v4
+      with:
+        repository: model-checking/kani
+        path: kani-tmp
 
     - name: Fetch Kani toolchain version
       run: |
-        mkdir kani-tmp
         cd kani-tmp
-        git init
-        git remote add origin https://github.com/model-checking/kani
-        git fetch --depth 1 origin main
-        git checkout main
         TOOLCHAIN_DATE=$(grep -oP 'channel = "nightly-\K\d{4}-\d{2}-\d{2}' rust-toolchain.toml)
         COMMIT_HASH=$(curl https://static.rust-lang.org/dist/$TOOLCHAIN_DATE/channel-rust-nightly-git-commit-hash.txt)
         if [ -z "$COMMIT_HASH" ]; then