Update benchmark.yml

Use ubuntu-20.04 instead of Ubuntu-latest b/c of issues with MINSIGSTKSZ (see https://stackoverflow.com/questions/71454588/minsigstksz-error-after-update-in-my-manjaro-linux)

.github/workflows/benchmark.yml

@@ -21,7 +21,7 @@ on:
 jobs:
   benchmarks:
     name: Run C++ benchmark
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}