Mark PBS & pyenv provider test as "platform-specific behaviour"

[huonw]

Experiment to see whether our existing tests catch the bug in the PBS provider on macOS (see #21551).

[huonw]

Yay, this does successfully catch the problem:

E       pants.engine.internals.scheduler.ExecutionError: 1 Exception encountered:
...
E       pants.engine.process.ProcessExecutionFailure: Process 'Install Python 3.9.10' failed with exit code 64.
E       stdout:
E       
E       stderr:
E       /bin/cp: illegal option -- -
E       usage: cp [-R [-H | -L | -P]] [-fi | -n] [-aclpsvXx] source_file target_file
E              cp [-R [-H | -L | -P]] [-fi | -n] [-aclpsvXx] source_file ... target_directory
...
FAILED src/python/pants/backend/python/providers/python_build_standalone/rules_integration_test.py::test_using_pbs[3.9.10]

[huonw]

Done more extensively in #21610