ci(desktop): cache vlc-setup downloads, drop skipVlcSetup bypass

Replace the -PskipVlcSetup=true CI bypass with a per-OS GitHub Actions
cache of ~/.gradle/vlcSetup. Cache key is hashFiles('desktopApp/build.gradle.kts')
so a VLC version bump invalidates cleanly; restore-keys lets unrelated
edits to that file still warm-start.

Cache hit: vlcDownload / upxDownload are up-to-date and we never touch
get.videolan.org. Cache miss (first run after version bump, or a new
runner): we fall back to the network, where the existing 5-attempt
retry budget in build.gradle.kts already handles flakes.

The skipVlcSetup property + AMETHYST_SKIP_VLC env hooks stay in
desktopApp/build.gradle.kts as a local-dev / CCW escape hatch — the
CCW egress can't reach get.videolan.org at all and has no actions/cache.

.github/workflows/build.yml

@@ -65,13 +65,23 @@ jobs:
           java-version: 21
           cache: gradle
 
-      # `-PskipVlcSetup=true` disables the vlcDownload / upxDownload / vlcSetup
-      # tasks (see desktopApp/build.gradle.kts). PR CI artifacts are smoke-test
-      # only — get.videolan.org is too flaky from GitHub-hosted runners to gate
-      # PRs on. Release packaging (create-release.yml) deliberately omits this
-      # flag so shipped artifacts still bundle VLC.
+      # Cache vlc-setup plugin downloads (VLC + UPX archives) keyed on the
+      # versions pinned in desktopApp/build.gradle.kts. Each OS gets its own
+      # cache namespace because the plugin downloads platform-specific archives.
+      # On a hit the vlcDownload / upxDownload tasks are up-to-date and we
+      # never touch get.videolan.org; on a miss (version bump or new runner)
+      # we fall back to fetching, which is what the in-build retry budget
+      # exists for.
+      - name: Cache vlc-setup downloads
+        uses: actions/cache@v4
+        with:
+          path: ~/.gradle/vlcSetup
+          key: vlcsetup-${{ runner.os }}-${{ hashFiles('desktopApp/build.gradle.kts') }}
+          restore-keys: |
+            vlcsetup-${{ runner.os }}-
+
       - name: Test + Build Desktop (gradle)
-        run: ./gradlew :quartz:jvmTest :commons:jvmTest :nestsClient:jvmTest :cli:test :desktopApp:test :desktopApp:${{ matrix.desktop-task }} --no-daemon -PskipVlcSetup=true
+        run: ./gradlew :quartz:jvmTest :commons:jvmTest :nestsClient:jvmTest :cli:test :desktopApp:test :desktopApp:${{ matrix.desktop-task }} --no-daemon
 
       # jpackage pins libicu to the build host's version (libicu74 on
       # ubuntu-24.04). Rewrite the .deb so testers on other Debian/Ubuntu