[libc++] Always run Ninja through xcrun in the macOS CI scripts

Ninja isn't installed by default on OSX, so run it through xcrun to find
the one in the developer tools if needed.

Author: ldionne

libcxx/utils/ci/macos-trunk.sh

@@ -109,18 +109,18 @@ echo "@@@@@@"
 
 
 echo "@@@ Building libc++.dylib and libc++abi.dylib from sources (just to make sure it works) @@@"
-ninja -C "${LLVM_BUILD_DIR}" install-cxx install-cxxabi -v
+xcrun ninja -C "${LLVM_BUILD_DIR}" install-cxx install-cxxabi -v
 echo "@@@@@@"
 
 
 echo "@@@ Running tests for libc++ @@@"
 # TODO: We should run check-cxx-abilist too
-ninja -C "${LLVM_BUILD_DIR}" check-cxx
+xcrun ninja -C "${LLVM_BUILD_DIR}" check-cxx
 echo "@@@@@@"
 
 
 echo "@@@ Running tests for libc++abi @@@"
-ninja -C "${LLVM_BUILD_DIR}" check-cxxabi
+xcrun ninja -C "${LLVM_BUILD_DIR}" check-cxxabi
 echo "@@@@@@"