fix(ci): persist vcpkg binary archives so qtbase stops rebuilding

The Qt cache hit every run yet qtbase was still compiled from source
(~32 min) each time. Two root causes:

1. The cache path included vcpkg\installed, but the build runs
   `gradlew clean` which deletes native/build/vcpkg_installed (the
   manifest-mode install dir). vcpkg restores packages from the
   file-backed binary cache (vcpkg-archives), which is the only thing
   that lets it skip a rebuild — and it restored 0 packages.

2. vcpkg-archives was never persisted across runs: the cache key was
   static (hash of vcpkg.json, which rarely changes), and actions/cache
   never overwrites an existing key. So every run got a cache hit on
   restore (empty archives), rebuilt qtbase, repopulated vcpkg-archives,
   then hit "cache hit, not saving" at Post — throwing the rebuilt
   archive away. Infinite rebuild loop.

Fix: cache only vcpkg-archives (drop the ~1.2 GB dead vcpkg\installed),
and rotate the key with github.run_id so Post always saves a fresh entry,
with layered restore-keys to warm-start from the newest prior archive.
The next build still primes the archive once (~32 min); subsequent builds
restore qtbase and finish in minutes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/build-loader.yml

@@ -110,20 +110,30 @@ 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
+      # Cache the vcpkg *binary archives* — the only thing that lets vcpkg skip
+      # rebuilding qtbase. The build runs `gradlew clean`, which deletes
+      # native/build/ (and thus the manifest install dir native/build/vcpkg_installed),
+      # so on every run vcpkg re-resolves the manifest and restores packages from
+      # this file-backed binary cache. If it's empty, qtbase is compiled from
+      # source (~32 min). (We previously cached vcpkg\installed too, but in
+      # manifest mode that tree is never consulted for the rebuild decision — only
+      # the binary archive is — so it was ~1.2 GB of dead weight.)
+      #
+      # The key MUST rotate per run: actions/cache never overwrites an existing
+      # key, so a static key froze whatever archives the first run captured (none)
+      # and every later run hit "cache hit, not saving" at Post — discarding the
+      # qtbase archive it had just rebuilt, guaranteeing a fresh 32-min rebuild
+      # forever. Appending run_id makes Post always save a new entry; the layered
+      # restore-keys warm-start from the newest prior archive so qtbase is
+      # restored, not rebuilt. (GitHub LRU-evicts old entries within the 10 GB
+      # budget; the archive is only a few hundred MB.)
+      - name: Cache vcpkg binary archives
         uses: actions/cache@v4
         with:
-          path: |
-            vcpkg\installed
-            vcpkg-archives
-          key: vcpkg-qt-${{ runner.os }}-${{ hashFiles('native/vcpkg.json') }}
+          path: vcpkg-archives
+          key: vcpkg-qt-${{ runner.os }}-${{ hashFiles('native/vcpkg.json') }}-${{ github.run_id }}
           restore-keys: |
+            vcpkg-qt-${{ runner.os }}-${{ hashFiles('native/vcpkg.json') }}-
             vcpkg-qt-${{ runner.os }}-
 
       - name: Install UPX