ci: Bump clang/LLVM version on Ubuntu 20.04

Due to recent changes in pid_iter.bpf.c, we need support for the
__builtin_preserve_enum_value build-in, which is not available in the
version that ships on Ubuntu 20.04 runners (clang 11). Let's upgrade it.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

.github/workflows/build.yaml

@@ -32,9 +32,17 @@ jobs:
         with:
           submodules: true
 
+      - name: Update APT repositories
+        run: sudo apt-get update
+
+      - name: Upgrade clang for older Ubuntu
+        if: matrix.os == 'ubuntu-20.04'
+        run: |
+          sudo apt-get remove -y clang-11 llvm-11
+          sudo apt-get install -y clang-12 llvm-12 llvm-12-dev
+
       - name: Install dependencies
         run: |
-          sudo apt-get update
           sudo apt-get install -y \
               libbfd-dev libcap-dev libelf-dev libiberty-dev python3-docutils
           # clang/LLVM are already installed, but we're missing some aliases.