fix: use find instead of ls for clang detection in CI

ls returns exit code 2 when glob has no matches, which fails the
build step under set -e + pipefail. find returns 0 even when no
files are found.

.gitea/workflows/ci.yml

@@ -38,9 +38,10 @@ jobs:
 
       - name: Build kappa
         run: |
-          CC=$(ls /usr/bin/clang-1[5-9] /usr/bin/clang-2[0-9] 2>/dev/null | head -1)
+          CC=$(find /usr/bin -maxdepth 1 \( -name 'clang-1[5-9]' -o -name 'clang-2[0-9]' \) 2>/dev/null | head -1)
           if [ -z "$CC" ]; then CC=/usr/bin/clang-18; fi
           CXX="${CC}++"
+          echo "Using compiler: $CXX"
           cmake -B build -G Ninja \
             -DCMAKE_C_COMPILER="$CC" \
             -DCMAKE_CXX_COMPILER="$CXX"