[libc++] Ensure the CI scripts can find Ninja

Ninja isn't necessarily installed on macOS out of the box, so make sure
we use the SDK Ninja instead of relying on one being available on the
system.

libcxx/utils/ci/macos-backdeployment.sh

@@ -95,6 +95,7 @@ mkdir -p "${LLVM_BUILD_DIR}"
   xcrun cmake \
     -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \
     -GNinja \
+    -DCMAKE_MAKE_PROGRAM="$(xcrun --find ninja)" \
     -DCMAKE_INSTALL_PREFIX="${LLVM_INSTALL_DIR}" \
     -DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi" \
     -DCMAKE_OSX_ARCHITECTURES="x86_64" \

libcxx/utils/ci/macos-trunk.sh

@@ -95,6 +95,7 @@ mkdir -p "${LLVM_BUILD_DIR}"
   xcrun cmake \
     -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \
     -GNinja \
+    -DCMAKE_MAKE_PROGRAM="$(xcrun --find ninja)" \
     -DCMAKE_INSTALL_PREFIX="${LLVM_INSTALL_DIR}" \
     -DLIBCXX_ENABLE_EXCEPTIONS="${LIBCXX_EXCEPTIONS}" \
     -DLIBCXXABI_ENABLE_EXCEPTIONS=ON \