[CI] Upload the vcpkg handover as one archive instead of loose files

.github/workflows/desktop-build.yml

@@ -451,6 +451,17 @@ jobs:
           prefix: ${{ matrix.vcpkg_scope }}
           token: ${{ secrets.GITHUB_TOKEN }}
 
+      # The action only creates its directory when the prefix matched at least one
+      # entry, and it reads that directory unguarded when it saves at the end of the
+      # job. A scope nobody has written to yet therefore leaves the directory missing
+      # and fails the job, which is exactly the state every scope starts in. Make the
+      # directory up front so an empty scope is a cache miss instead of an error.
+      - name: "Create vcpkg cache directory"
+        shell: bash
+        env:
+          CACHE_DIR: '${{ steps.vcpkg-cache.outputs.path }}'
+        run: mkdir -p "$CACHE_DIR"
+
       # Uses environment variables, see compile.sh for more details
       - name: "Build Cockatrice"
         id: build
@@ -470,12 +481,15 @@ jobs:
           PACKAGE_SUFFIX: '${{ matrix.package_suffix }}'
           TARGET_MACOS_VERSION: ${{ matrix.override_target }}
           USE_CCACHE: ${{ matrix.use_ccache }}
-          # Only master may write the cache itself. A pull request run has its own
-          # merge ref scope, which nothing else can read, so anything it saved there
-          # would be unreachable for every later run. Handing the new archives over
-          # to vcpkg-cache-import.yml is what lets a pull request heal the shared
-          # cache after a runner image update changes every ABI hash.
-          VCPKG_BINARY_SOURCES: "clear;files,${{ steps.vcpkg-cache.outputs.path }},${{ case(github.ref == 'refs/heads/master', 'readwrite', 'read') }}"
+          # vcpkg has to be allowed to write into the cache directory even on a pull
+          # request, because that is where "Collect new vcpkg entries" looks for the
+          # archives this run had to compile itself. With the source left at read those
+          # archives are never written, the collect step finds nothing, and vcpkg-cache-import.yml
+          # gets no artifact to heal the shared cache with. The importer is what publishes
+          # to master's scope; a pull request's own merge ref stays unreadable by every
+          # later run, so the action's own save there costs quota but nothing else.
+          # Forks keep the read-only mode, they are the case that must not be trusted.
+          VCPKG_BINARY_SOURCES: "clear;files,${{ steps.vcpkg-cache.outputs.path }},${{ case(github.ref == 'refs/heads/master' || (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository), 'readwrite', 'read') }}"
           VCPKG_DISABLE_METRICS: 1
           VCPKG_FEATURE_FLAGS: dependencygraph
           VCPKG_TARGET_MACOS_VERSION: ${{ matrix.vcpkg_deployment_target }}
@@ -532,12 +546,15 @@ jobs:
           echo "count=$count" >>"$GITHUB_OUTPUT"
           echo "slug=$(sed -e 's|^vcpkg/||' -e 's|/$||' -e 's|/|-|g' <<<"$VCPKG_SCOPE")" >>"$GITHUB_OUTPUT"
 
+      # archive: false accepts a single file and nothing else, and a scope hands over one
+      # archive per ABI hash vcpkg had to rebuild, which is 23 files on a cold runner.
+      # Zipping the directory keeps the fanout at the artifact root, which is where
+      # vcpkg-cache-import.yml looks for keys.txt once download-artifact unpacks it.
       - name: "Upload new vcpkg entries"
         # An empty count means the step above did not run at all, on master and on forks
         if: steps.vcpkg_new.outputs.count != '' && steps.vcpkg_new.outputs.count != '0'
         uses: actions/upload-artifact@v7
         with:
-          archive: false
           if-no-files-found: error    # a count above zero with nothing staged is a bug
           name: vcpkg-new-${{ steps.vcpkg_new.outputs.slug }}
           path: ${{ runner.temp }}/vcpkg-new