ci: stop re-downloading CMake/Ninja on every Windows run

lukka/get-cmake@latest re-fetched and re-extracted CMake + Ninja on
every single Windows CI run. Its own cache (routed through this
act_runner's built-in cache server) reported "Cloud cache miss" on
every run, even one immediately after a run that logged a successful
save under the exact same key -- an incompatibility between its
bundled cache client and this act_runner's cache-server implementation,
not a config gap. Separately and more importantly: the archive
extraction step alone measured ~7.5 minutes for a 45MB zip on this VM,
consistent with Defender real-time scanning rather than raw disk I/O.
Together this was the dominant cost of every Windows CI run.

CMake 4.4.2 and Ninja 1.12.1 are now installed once, directly on the
winvm-builder VM's system PATH (C:\BuildTools\cmake, C:\BuildTools\
ninja), sidestepping the third-party action's cache entirely rather
than debugging its internals further. Both Windows jobs now just
verify cmake/ninja are present and fail loudly if not, instead of
silently falling back to a slow reinstall. Full detail, including how
to redo this if the VM is ever rebuilt, is in README's new "Windows
runner: persistent build tools" section -- this is VM state, not
something git reproduces.

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

.gitea/workflows/build.yml

@@ -127,11 +127,29 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Install build dependencies
+      - name: Verify build dependencies
         # winvm-builder is a self-hosted act_runner labeled "windows-latest";
         # it is NOT the GitHub-hosted image, so none of that image's
-        # preinstalled tooling (cmake included) can be assumed present.
-        uses: lukka/get-cmake@latest
+        # preinstalled tooling can be assumed present. This used to be
+        # `uses: lukka/get-cmake@latest`, which re-downloaded and
+        # re-extracted CMake + Ninja on every single run -- its own cache
+        # (routed through this act_runner's cache server) reported a "cloud
+        # cache miss" on every run even immediately after a successful save,
+        # and separately the extraction step alone measured ~7.5 minutes on
+        # this VM (consistent with Defender real-time scanning, not raw I/O)
+        # -- together the dominant cost of every Windows CI run. CMake and
+        # Ninja are now installed once, directly on winvm-builder's system
+        # PATH (C:\BuildTools\cmake\bin, C:\BuildTools\ninja -- see the
+        # README's "Windows runner: persistent build tools" section for
+        # exactly what that machine has installed and how to redo it if the
+        # VM is ever rebuilt). This step just fails loudly if that ever
+        # stops being true, rather than silently falling back to a slow
+        # re-download.
+        shell: powershell
+        run: |
+          $ErrorActionPreference = "Stop"
+          cmake --version
+          ninja --version
 
       - name: Cache OBS SDK bootstrap deps
         # See the matching step in the macOS job above for why this is