Use easy way to install clang-20

Author: bashtage

ci/azure/azure_template_posix.yml

@@ -112,13 +112,9 @@ jobs:
     condition: ne(variables['use.conda'], 'true')
 
   - script: |
-     sudo apt-get update
-     sudo apt-get install -y wget software-properties-common
-     sudo wget -qO - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - 
-     sudo add-apt-repository "deb http://apt.llvm.org/noble/ llvm-toolchain-noble main" -y
-     sudo apt-get update
-     sudo apt-get install -y clang-20
-    displayName: 'Install Clang-21'
+      sudo apt update
+      sudo apt install clang-20 lldb-20 clangd-20
+    displayName: 'Install Clang'
     condition: eq(variables['use.clang'], 'true')
 
   - script: |