ci: switch to Arch Linux container for modern toolchain

Debian 11's libstdc++ (GCC 10) lacks <format> even with clang-18.
Arch ships clang-18 and the latest libstdc++ out of the box.
Container-based runner with pacman for dead-simple dependency install.

.gitea/workflows/ci.yml

@@ -9,30 +9,20 @@ on:
 jobs:
   build-and-test:
     runs-on: ubuntu-latest
+    container: archlinux:latest
     steps:
       - uses: actions/checkout@v4
 
       - name: Install dependencies
         run: |
-          apt-get update -qq
-          apt-get install -y -qq wget lsb-release software-properties-common gnupg
-          wget -qO- https://apt.llvm.org/llvm.sh | bash -s -- 18 all
-          apt-get install -y -qq ninja-build curl
-
-      - name: Install CMake 3.28
-        run: |
-          curl -sL https://github.com/Kitware/CMake/releases/download/v3.28.6/cmake-3.28.6-linux-x86_64.tar.gz | tar xz -C /usr/local --strip-components=1
-          cmake --version
+          pacman -Syu --noconfirm
+          pacman -S --noconfirm clang cmake ninja
 
       - name: Build kappa
         run: |
-          CC=$(find /usr/bin -maxdepth 1 \( -name 'clang-1[5-9]' -o -name 'clang-2[0-9]' \) 2>/dev/null | sort -V | tail -1)
-          if [ -z "$CC" ]; then CC=/usr/bin/clang-18; fi
-          CXX=$(echo "$CC" | sed 's/clang-/clang++-/')
-          echo "CC=$CC CXX=$CXX"
           cmake -B build -G Ninja \
-            -DCMAKE_C_COMPILER="$CC" \
-            -DCMAKE_CXX_COMPILER="$CXX"
+            -DCMAKE_C_COMPILER=clang \
+            -DCMAKE_CXX_COMPILER=clang++
           cmake --build build
 
       - name: DSL and CLI tests