[test] Exclude //llvm/unittests:llvm_exegesis_tests due to buildkite environment.

Buildkite does not allow user perf monitoring and fails: https://buildkite.com/llvm-project/upstream-bazel/builds/49579.

```
[ RUN      ] PerfHelperTest.FunctionalTest
Unable to open event. ERRNO: Permission denied. Make sure your kernel allows user space perf monitoring.
You may want to try:
$ sudo sh -c 'echo -1 > /proc/sys/kernel/perf_event_paranoid'
llvm_exegesis_tests: external/llvm-project/llvm/tools/llvm-exegesis/lib/PerfHelper.cpp:111: llvm::exegesis::pfm::Counter::Counter(llvm::exegesis::pfm::PerfEvent &&): Assertion `FileDescriptor != -1 && "Unable to open event"' failed.
```