fix: install CMake 3.28 in CI (runner has 3.18, we need 3.20+)

Download official CMake binary from Kitware releases instead of
relying on the distro package manager which provides an older version.

.gitea/workflows/ci.yml

@@ -15,7 +15,12 @@ jobs:
       - name: Install dependencies
         run: |
           apt-get update -qq
-          apt-get install -y -qq clang cmake ninja-build
+          apt-get install -y -qq clang 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
 
       - name: Build kappa
         run: |