CI: switch vcpkg cache from removed x-gha backend to actions/cache

The first attempt used VCPKG_BINARY_SOURCES=x-gha, which vcpkg has since
removed (the configure step emitted "binary caching backend has been
removed"). Replaced with actions/cache on %LOCALAPPDATA%\vcpkg\archives,
keyed on vcpkg.json. Same effective speedup, no GHA service tokens needed.

Author: meta-legend

.github/workflows/ci.yml

@@ -32,21 +32,22 @@ jobs:
         if: runner.os != 'Windows'
         run: cmake -B build -S . -DCMAKE_BUILD_TYPE=Release
 
-      # Expose the GitHub Actions cache service tokens so vcpkg's x-gha binary
-      # cache can read/write built packages across runs. These env vars aren't
-      # forwarded into job steps by default; github-script re-exports them.
-      - name: Export GitHub Actions cache env (Windows)
+      # Cache vcpkg's built-package archives so curl/nlohmann-json are not
+      # rebuilt from source on every run. vcpkg's old x-gha backend was
+      # removed; using actions/cache on the archives directory gives the same
+      # speedup without provider setup. Key on vcpkg.json so a dep bump
+      # invalidates cleanly.
+      - name: Cache vcpkg archives (Windows)
         if: runner.os == 'Windows'
-        uses: actions/github-script@v7
+        uses: actions/cache@v4
         with:
-          script: |
-            core.exportVariable('ACTIONS_CACHE_URL', process.env.ACTIONS_CACHE_URL || '');
-            core.exportVariable('ACTIONS_RUNTIME_TOKEN', process.env.ACTIONS_RUNTIME_TOKEN || '');
+          path: ~\AppData\Local\vcpkg\archives
+          key: vcpkg-${{ runner.os }}-${{ hashFiles('vcpkg.json') }}
+          restore-keys: |
+            vcpkg-${{ runner.os }}-
 
       - name: Configure (Windows, vcpkg)
         if: runner.os == 'Windows'
-        env:
-          VCPKG_BINARY_SOURCES: "clear;x-gha,readwrite"
         run: |
           # Sync the runner's vcpkg to the manifest's builtin-baseline so its
           # port/version database matches, then configure.