[libc++] Switch FreeBSD CI job to Clang 17

libc++ will drop support for Clang 16 before long.

Author: emaste

libcxx/utils/ci/buildkite-pipeline.yml

@@ -209,8 +209,8 @@ steps:
   - label: FreeBSD 13 amd64
     command: libcxx/utils/ci/run-buildbot generic-cxx23
     env:
-      CC: clang16
-      CXX: clang++16
+      CC: clang17
+      CXX: clang++17
     agents:
       queue: libcxx-builders
       os: freebsd