fix: remove debian LLVM PPA repo (Linux runners only)

resolves #265

Users that expect the PPA to be present _after_ running cpp-linter-action can set the environment variable `KEEP_DEBIAN_LLVM_PPA` to `true`, `on`, or `1`. By default, the PPA is removed.

action.yml

@@ -249,6 +249,12 @@ runs:
           chmod +x $GITHUB_ACTION_PATH/llvm_install.sh
           if sudo $GITHUB_ACTION_PATH/llvm_install.sh ${{ inputs.version }}; then
             sudo apt-get install -y clang-format-${{ inputs.version }} clang-tidy-${{ inputs.version }}
+            case "${{ env.KEEP_DEBIAN_LLVM_PPA }}" in
+              "true" | "on" | "1");;
+              *)
+                sudo rm /etc/apt/sources.list.d/*llvm*.list || true
+                ;;
+            esac
           fi
         fi