fuzz: explicitly specify llvm-symbolizer path in runner

It's not completely clear to me why this needs to be explicitly
specified in some environments, and not in others, while at the same time
that `llvm-symbolizer` is already in PATH, but this has fixed the 2 issues
outlined in #28147.

Use `LLVM_SYMBOLIZER_PATH` as the env var, as that is somewhat also used
inside LLVM, but not consistently, i.e it's checked for in the asan_symbolize
script, but not in in the ubsan_symbolize script, or from in compiler-rt.

test/fuzz/test_runner.py

@@ -16,11 +16,14 @@ import sys
 
 
 def get_fuzz_env(*, target, source_dir):
+    symbolizer = os.environ.get('LLVM_SYMBOLIZER_PATH', "/usr/bin/llvm-symbolizer")
     return {
         'FUZZ': target,
         'UBSAN_OPTIONS':
         f'suppressions={source_dir}/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=1:report_error_type=1',
+        'UBSAN_SYMBOLIZER_PATH':symbolizer,
         "ASAN_OPTIONS": "detect_stack_use_after_return=1:check_initialization_order=1:strict_init_order=1",
+        'ASAN_SYMBOLIZER_PATH':symbolizer,
     }