ci: install cmake on the Windows runner before configuring

First CI run confirmed winvm-builder is a self-hosted act_runner
labeled "windows-latest" but not the GitHub-hosted windows-latest
image -- cmake isn't on PATH there (unlike the Linux/macOS runners,
which do have working system/brew package managers). Use
lukka/get-cmake to fetch a pinned cmake+ninja without needing
admin/choco.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

.gitea/workflows/build.yml

@@ -60,6 +60,16 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Install build dependencies
+        # winvm-builder is a self-hosted act_runner labeled
+        # "windows-latest" -- it is NOT the GitHub-hosted windows-latest
+        # image, so none of the tools that image preinstalls (cmake
+        # included) can be assumed present. Confirmed by a first CI run
+        # on this repo: "cmake : The term 'cmake' is not recognized...".
+        # lukka/get-cmake downloads a pinned cmake+ninja binary and adds
+        # it to PATH for this job, with no admin/choco dependency.
+        uses: lukka/get-cmake@latest
+
       - name: Configure
         # Same story as macOS: no OBS SDK available yet on this runner,
         # so this proves the core library + MSVC toolchain only.