ci(loader): cache vcpkg installed/ via actions/cache, drop x-gha

The previous run still rebuilt Qt from source on the second push.
vcpkg's x-gha binary cache backend relies on the v1 GitHub Actions
cache API and the ACTIONS_CACHE_URL / ACTIONS_RUNTIME_TOKEN
secrets exported via actions/github-script. On the current
windows-2022 image the v1 API is being phased out (the new
ACTIONS_RESULTS_URL is what's reachable), so x-gha quietly missed
on every run.

Switch to a path-based actions/cache@v4 entry:

  - path: vcpkg\installed + vcpkg-archives
  - key:  vcpkg-qt-${runner.os}-${hash(native/vcpkg.json)}
  - restore-keys: vcpkg-qt-${runner.os}- (warm start on a vcpkg.json bump)

vcpkg.installed is the deployed Qt tree CMake's manifest mode
inspects; if it already contains qtbase[widgets] the first vcpkg
install call is a no-op and CMake configure finishes in seconds.
vcpkg-archives gets fed via the new
VCPKG_BINARY_SOURCES=clear;files,...\vcpkg-archives,readwrite
binary source so transitive package binaries are also persisted
across runs - useful when vcpkg.json changes and only some libs
need rebuilding.

Also drop the now-unused "Export GitHub Actions cache tokens"
step (no x-gha backend, no tokens needed).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/build-loader.yml

@@ -22,7 +22,15 @@ jobs:
       # wins over a job-level env in PowerShell expansions. We export
       # the right VCPKG_ROOT into $GITHUB_ENV inside the clone step
       # so every later step sees our copy.
-      VCPKG_BINARY_SOURCES: "clear;x-gha,readwrite"
+      #
+      # We rely on actions/cache for the installed Qt artifacts; the
+      # vcpkg x-gha backend depends on the v1 GitHub Actions cache API
+      # which has been deprecated, and a fresh runner image often does
+      # not have ACTIONS_RESULTS_URL set for non-Node actions yet.
+      # Setting "files" as the binary source means vcpkg will still
+      # populate ${VCPKG_ROOT}/archives, which actions/cache then
+      # snapshots along with installed/.
+      VCPKG_BINARY_SOURCES: "clear;files,${{ github.workspace }}\\vcpkg-archives,readwrite"
 
     steps:
       - name: Checkout
@@ -49,17 +57,6 @@ jobs:
         with:
           arch: x64
 
-      # Expose ACTIONS_CACHE_URL / ACTIONS_RUNTIME_TOKEN to subsequent
-      # steps so vcpkg's x-gha cache backend works. These are set by
-      # the runner but only exposed to actions via the toolkit; we
-      # forward them manually for the shell-based vcpkg calls below.
-      - name: Export GitHub Actions cache tokens
-        uses: actions/github-script@v7
-        with:
-          script: |
-            core.exportVariable('ACTIONS_CACHE_URL', process.env.ACTIONS_CACHE_URL || '');
-            core.exportVariable('ACTIONS_RUNTIME_TOKEN', process.env.ACTIONS_RUNTIME_TOKEN || '');
-
       - name: Clone & bootstrap vcpkg
         shell: pwsh
         run: |
@@ -74,6 +71,22 @@ jobs:
           "VCPKG_ROOT=$vcpkg" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
           Write-Host "Exported VCPKG_ROOT=$vcpkg to GITHUB_ENV"
 
+      # Cache the resolved Qt install + the file-backed binary archive.
+      # Key is keyed on vcpkg.json so any dependency change invalidates;
+      # restore-keys lets a vcpkg.json bump still warm-start from the
+      # previous build's archives. installed/ alone makes CMake's manifest
+      # mode skip rebuilding qtbase; archives/ also fills back transitive
+      # deps if a partial rebuild is needed.
+      - name: Cache vcpkg installed + archives
+        uses: actions/cache@v4
+        with:
+          path: |
+            vcpkg\installed
+            vcpkg-archives
+          key: vcpkg-qt-${{ runner.os }}-${{ hashFiles('native/vcpkg.json') }}
+          restore-keys: |
+            vcpkg-qt-${{ runner.os }}-
+
       - name: Install UPX
         shell: pwsh
         run: choco install upx -y --no-progress