ci: cache the OBS SDK bootstrap deps across runs

cmake/{windows,macos}/buildspec.cmake download the pinned obs-deps
bundle + obs-studio source into .deps/, with its own idempotent
skip-if-present logic keyed on SHA256 marker files. That logic never
gets a chance to fire because .deps/ lives inside the checkout and a
fresh `actions/checkout` wipes it on every single push -- this is what
has made every CI iteration tonight re-pay the several-minute cold
download instead of only paying it once.

Add an actions/cache step for .deps/ in both jobs, keyed on the files
that actually pin dependency versions (buildspec.json +
buildspec_common.cmake + the platform buildspec.cmake), so a version
bump still invalidates the cache correctly. No runner-side config
change needed -- the act_runner instance already has its built-in
cache server on by default (cache.enabled: true, confirmed via
`gitea-runner.exe generate-config` on winvm-builder).

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

.gitea/workflows/build.yml

@@ -69,6 +69,19 @@ jobs:
       - name: Install build dependencies
         run: .gitea/scripts/macos-deps.sh
 
+      - name: Cache OBS SDK bootstrap deps
+        # cmake/macos/buildspec.cmake downloads the pinned obs-deps bundle +
+        # obs-studio source into .deps/ and already skips re-downloading via
+        # SHA256 marker files -- but .deps/ lives inside the checkout, so a
+        # fresh `actions/checkout` above wipes it every run regardless. This
+        # cache step is what actually makes that existing skip-logic apply
+        # across runs. The runner's built-in cache server is on by default
+        # (cache.enabled: true), so this needs no runner-side config.
+        uses: actions/cache@v4
+        with:
+          path: .deps
+          key: obs-deps-${{ runner.os }}-${{ hashFiles('cmake/macos/buildspec.cmake', 'cmake/common/buildspec_common.cmake', 'buildspec.json') }}
+
       - name: Configure, build, test, verify
         run: .gitea/scripts/macos-build.sh
 
@@ -92,6 +105,16 @@ jobs:
         # preinstalled tooling (cmake included) can be assumed present.
         uses: lukka/get-cmake@latest
 
+      - name: Cache OBS SDK bootstrap deps
+        # See the matching step in the macOS job above for why this is
+        # needed: cmake/windows/buildspec.cmake's own download logic is
+        # already idempotent, it just never gets the chance because .deps/
+        # lives inside the checkout and is wiped by every fresh clone.
+        uses: actions/cache@v4
+        with:
+          path: .deps
+          key: obs-deps-${{ runner.os }}-${{ hashFiles('cmake/windows/buildspec.cmake', 'cmake/common/buildspec_common.cmake', 'buildspec.json') }}
+
       - name: Configure, build, test, verify
         # Windows PowerShell (powershell.exe), not PowerShell Core (pwsh) --
         # this self-hosted runner does not have pwsh installed, only the