Update CI

Signed-off-by: FedericoBruzzone <federico.bruzzone.i@gmail.com>

Author: FedericoBruzzone

.github/workflows/ubuntu-x86.yml

@@ -24,19 +24,22 @@ jobs:
           CC: ${{ matrix.compiler.CC }}
           CXX: ${{ matrix.compiler.CXX }}
         run: |
-          wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key | sudo gpg --dearmor -o /usr/share/keyrings/llvm-keyring.gpg
+          sudo apt-get update -y
+          sudo apt-get install -y --no-install-recommends ca-certificates wget python3-pip python3-setuptools
 
-          echo "deb [signed-by=/usr/share/keyrings/llvm-keyring.gpg] http://apt.llvm.org/jammy/ llvm-toolchain-jammy-21 main" | sudo tee /etc/apt/sources.list.d/llvm.list
+          # Install LLVM using the official apt.llvm.org installer script
+          wget -q https://apt.llvm.org/llvm.sh
+          chmod +x llvm.sh
+          sudo ./llvm.sh 21
 
-          sudo apt-get update
-          sudo apt-get install -y --fix-missing \
-            llvm-21 \
-            llvm-21-dev \
-            llvm-21-tools \
-            clang-21 \
-            python3-setuptools
+          # apt.llvm.org is snapshot-based; packages referenced by the current index can rotate and 404.
+          # Mitigation: retries + refresh indexes and retry the install once.
+          if ! sudo apt-get -o Acquire::Retries=5 install -y --no-install-recommends llvm-21 llvm-21-dev llvm-21-tools clang-21; then
+            sudo apt-get update -y
+            sudo apt-get -o Acquire::Retries=5 install -y --no-install-recommends llvm-21 llvm-21-dev llvm-21-tools clang-21
+          fi
 
-          sudo pip3 install lit
+          sudo pip3 install --no-cache-dir lit
       - name: Build `llvm-pass-template`
         run: |
           cd $GITHUB_WORKSPACE