[libc++] Use xcrun to find Ninja in the macOS backdeployment CI too

GitOrigin-RevId: f1a987366fb9e20abdcb5f87e2be1dabb4401cb2
diff --git a/utils/ci/macos-backdeployment.sh b/utils/ci/macos-backdeployment.sh
index b511faf..24b866c 100755
--- a/utils/ci/macos-backdeployment.sh
+++ b/utils/ci/macos-backdeployment.sh
@@ -105,7 +105,7 @@
 
 
 echo "@@@ Building and installing libc++ and libc++abi @@@"
-ninja -C "${LLVM_BUILD_DIR}" install-cxx install-cxxabi
+xcrun ninja -C "${LLVM_BUILD_DIR}" install-cxx install-cxxabi
 echo "@@@@@@"