Add `llvm-symbolizer` to PATH for `Centipede`

[DonggeLiu]

Addresses #12124. Unlikely local test runs, cloud VM cannot find llvm-symbolizer; this PR adds it to /bin to ensure Centipede can use it.

[DonggeLiu]

I am not sure if the error is related to my commit: It's about libFuzzer. Can we skip it?

Traceback (most recent call last):
  File "/workspace/src/clusterfuzz/_internal/tests/core/bot/fuzzers/libFuzzer/engine_test.py", line 624, in test_minimize
    minimize_output_path, 120)
  File "/workspace/src/clusterfuzz/_internal/bot/fuzzers/libFuzzer/engine.py", line 582, in minimize_testcase
    raise TimeoutError('Minimization timed out.')
TimeoutError: Minimization timed out.

I reckon centipede's test is done, according to the last line before the crash:

Done running ... tests.core.bot.fuzzers.centipede.centipede_engine_test, ...