fix: lower ubuntu runner version to 20.04

Author: Florian Gläßer

.github/workflows/build-llvm.yaml

@@ -13,7 +13,7 @@ jobs:
     name: Build LLVM
     strategy:
       matrix:
-        os: [ubuntu-24.04, ubuntu-24.04-arm, macos-14]
+        os: [ubuntu-20.04, ubuntu-24.04-arm, macos-14]
     runs-on: ${{ matrix.os }}
     steps:
       - name: checkout