Fix Python 3.12 deprecation warning about os.fork()

[mhucka]

In Python 3.12, on Linux at least, src/openfermion/testing/performance_benchmarks_test.py produces the following warnings:

~/.pyenv/versions/3.12.5/lib/python3.12/multiprocessing/popen_fork.py:66: RuntimeWarning:
os.fork() was called. os.fork() is incompatible with multithreaded code, and JAX is
multithreaded, so this will likely lead to a deadlock.
self.pid = os.fork()
src/openfermion/testing/performance_benchmarks_test.py::test_run_linear_qop
src/openfermion/testing/performance_benchmarks_test.py::test_run_linear_qop
src/openfermion/testing/performance_benchmarks_test.py::test_run_linear_qop
src/openfermion/testing/performance_benchmarks_test.py::test_run_linear_qop
src/openfermion/testing/performance_benchmarks_test.py::test_run_linear_qop
~/.pyenv/versions/3.12.5/lib/python3.12/multiprocessing/popen_fork.py:66: 
DeprecationWarning: This process (pid=3069778) is multi-threaded, use of fork() 
may lead to deadlocks in the child. self.pid = os.fork()

This can be resolved by calling multiprocessing.set_start_method() in __init__ of the ParallelLinearQuibitOperator class.

[fdmalone]

Seems like the default behaviour will change in python 3.14 (away from fork), maybe worth opening an issue and adding it as a link in the code to revisit in the future.