Try to fix benchmark comparison

Author: alexcrichton

.github/workflows/main.yml

@@ -78,6 +78,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+    - run: git fetch origin main && git branch main origin/main
     - run: (cd crates/wasmparser && ./compare-with-main.sh)
 
   fuzz: