[CFL] Interpret PARALLEL_FUZZING as a boolean value (#12655)

The `PARALLEL_FUZZING` environment variable is used to enable parallel
fuzzing in CFLite. The variable is always defined by GH Action, but it
was not interpreted as a boolean value, so it was always enabled even if
the option is set to `false`.

Co-authored-by: Vitor Guidi <[email protected]>

Author: kateinoigakukun

infra/cifuzz/config_utils.py

@@ -129,7 +129,7 @@ def __init__(self):
     self.build_integration_path = (
         constants.DEFAULT_EXTERNAL_BUILD_INTEGRATION_PATH)
 
-    self.parallel_fuzzing = os.environ.get('PARALLEL_FUZZING')
+    self.parallel_fuzzing = environment.get_bool('PARALLEL_FUZZING', False)
     self.extra_environment_variables = _get_extra_environment_variables()
     self.output_sarif = environment.get_bool('OUTPUT_SARIF', False)