Add CI to check that libfuzzer is vendored at the correct version

Avoids version mismatches, supply-chain attacks, etc...

.github/workflows/rust.yml

@@ -3,12 +3,22 @@ name: Rust
 on: [push, pull_request]
 
 jobs:
+  check_libfuzzer_checkout:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Re-vendor libfuzzer
+      run: ./update-libfuzzer.sh
+    - name: Check that nothing changed
+      run: git diff --exit-code
+
   build:
 
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v4
 
     - name: Install nightly Rust
       run: |