[CI] upgrade to ubuntu 22.04

Author: goodtaeeun

.github/workflows/build.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   check:
     name: build
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
@@ -36,10 +36,19 @@ jobs:
         run: |
           eval $(opam env)
           script/check-format
+
+      - name: Install LLVM 12
+        run: |
+          sudo apt-get update
+          echo “deb http://apt.llvm.org/focal/ llvm-toolchain-focal-12 main” | sudo tee /etc/apt/sources.list.d/llvm12.list
+          wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+          sudo apt-get update
+          sudo apt-get install -y llvm-12-dev libclang-12-dev clang-12
+      
       - name: Install deps
         run: |
           sudo apt-get update
-          sudo apt-get install -y libmpfr-dev llvm-12-dev pkg-config libclang-12-dev
+          sudo apt-get install -y libmpfr-dev pkg-config
 
       - name: Build Sparrow
         run: |