[CI] Build Windows releases with Ninja (#7380)

The A/B run measured Ninja against the Visual Studio generator across
ten builds: Ninja medians sit at ~17 min against ~21 for MSBuild, and
more importantly Ninja already matches the Linux and macOS legs, runs
the compiler itself and parallelizes the whole graph instead of one
MSBuild node at a time. Single runs on shared 4-core hosts swing by
minutes either way (VS and Ninja legs even ran in different Azure
regions inside one matrix), so this switches on medians and
engineering coherence, not on any one timing.

The Ninja leg is now the release leg: it keeps the -Win10 suffix and
the vcpkg scope (named after the vs2026 toolchain, whose cl.exe is
unchanged, so the warm archives keep restoring), uploads PDBs from the
single-config layout and ships installers to releases. The experiment
scaffolding goes away with the generator it measured.

.ci/compile.sh

@@ -314,26 +314,33 @@ if [[ $RUNNER_OS == macOS ]]; then
   fi
 
 elif [[ $RUNNER_OS == Windows ]]; then
-  # cmake only hands MSBuild a job count when one is asked for, and without it
-  # MSBuild builds a single node: one project at a time, whatever MTT does inside
-  # it. Ask for a node per core.
-  if [[ ! $BUILD_PARALLEL_LEVEL ]]; then
-    BUILD_PARALLEL_LEVEL="${NUMBER_OF_PROCESSORS:-1}"
-  fi
-  buildflags+=(--parallel "$BUILD_PARALLEL_LEVEL")
+  if [[ $CMAKE_GENERATOR == "Visual Studio"* ]]; then
+    # cmake only hands MSBuild a job count when one is asked for, and without it
+    # MSBuild builds a single node: one project at a time, whatever MTT does inside
+    # it. Ask for a node per core.
+    if [[ ! $BUILD_PARALLEL_LEVEL ]]; then
+      BUILD_PARALLEL_LEVEL="${NUMBER_OF_PROCESSORS:-1}"
+    fi
+    buildflags+=(--parallel "$BUILD_PARALLEL_LEVEL")
 
-  # Enable MTT, see https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/
-  # and https://devblogs.microsoft.com/cppblog/cpp-build-throughput-investigation-and-tune-up/#multitooltask-mtt
-  # EnforceProcessCountAcrossBuilds makes the CL_MPCount ceiling apply across all
-  # the projects the nodes are building, instead of each node running its own
-  # cl.exe processes and /m and MTT multiplying into an oversubscribed machine.
-  buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true -p:EnforceProcessCountAcrossBuilds=true -p:CL_MPCount="$BUILD_PARALLEL_LEVEL")
-  if [[ $BUILD_PROFILE ]]; then
-    # A performance summary says which projects and tasks cost what, the binary log
-    # keeps the full timeline; both are far too noisy to always produce. The files
-    # land next to the solution, i.e. in the build dir the workflow uploads. These
-    # join the MTT options above, which already carried the "--" separator.
-    buildflags+=("-flp:PerformanceSummary;v=q;LogFile=msbuild-perf.log" -bl:msbuild.binlog)
+    # Enable MTT, see https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/
+    # and https://devblogs.microsoft.com/cppblog/cpp-build-throughput-investigation-and-tune-up/#multitooltask-mtt
+    # EnforceProcessCountAcrossBuilds makes the CL_MPCount ceiling apply across all
+    # the projects the nodes are building, instead of each node running its own
+    # cl.exe processes and /m and MTT multiplying into an oversubscribed machine.
+    buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true -p:EnforceProcessCountAcrossBuilds=true -p:CL_MPCount="$BUILD_PARALLEL_LEVEL")
+    if [[ $BUILD_PROFILE ]]; then
+      # A performance summary says which projects and tasks cost what, the binary log
+      # keeps the full timeline; both are far too noisy to always produce. The files
+      # land next to the solution, i.e. in the build dir the workflow uploads. These
+      # join the MTT options above, which already carried the "--" separator.
+      buildflags+=("-flp:PerformanceSummary;v=q;LogFile=msbuild-perf.log" -bl:msbuild.binlog)
+    fi
+  else
+    # Ninja runs the compiler itself, so it neither takes the MSBuild switches
+    # (it rejects -p: options outright) nor a job count: it sizes its own pool
+    # for the cores it finds.
+    echo "ninja $(ninja --version)"
   fi
 fi