ci: trigger build only on tag push and workflow_dispatch, add cache for static ffmpeg

.gitea/workflows/build.yaml

@@ -2,8 +2,6 @@ name: Build Windows Installer
 
 on:
   push:
-    branches:
-      - main
     tags:
       - 'v*'
   workflow_dispatch:
@@ -74,8 +72,17 @@ jobs:
             -ldflags="-H windowsgui -s -w -X main.Version=${{ steps.ver.outputs.version }}" \
             -o dist/convert.exe .
 
+      - name: Cache static FFmpeg binaries
+        id: cache-ffmpeg
+        uses: actions/cache@v3
+        with:
+          path: dist/bin
+          key: ffmpeg-win64-master-static
+
       - name: Download static FFmpeg & FFprobe for Windows
+        if: steps.cache-ffmpeg.outputs.cache-hit != 'true'
         run: |
+          mkdir -p dist/bin
           wget -q -O ffmpeg.zip "https://github.com/BtbN/FFmpeg-Builds/releases/download/latest/ffmpeg-master-latest-win64-gpl.zip"
           unzip -j -q ffmpeg.zip "*/bin/ffmpeg.exe" "*/bin/ffprobe.exe" -d dist/bin/
           rm -f ffmpeg.zip