test: simplify test fixtures

Depends on release of aws-deadline/deadline-cloud-test-fixtures#164

The worker agent end-to-end tests had overridden the worker_config fixture from deadline-cloud-test-fixtures to work around bugs and enhance its functionality. This bug has been fixed upstream (see aws-deadline/deadline-cloud-test-fixtures#164)
The worker agent end-to-end tests had used a combination of:
```
pytest.mark.parametrize("operating_system", ["linux"], indirect=True)
pytest.mark.skipif(os.environ["OPERATING_SYSTEM"] == "windows")
```
These are redundant. We should not need to use both of the above for each test case.

Modify the worker_config fixture override to inherit the value from deadline-cloud-test-fixture but modify only the parts necessary
Modify the operating_system fixture to take its value from the OPERATING_SYSTEM environment variable

There is less duplication of the worker_config fixture from the upstream deadline-cloud-test-fixtures package and we do not have to maintain synchronization of their logic
Tests are simpler to write and do not need a combination of pytest parametrization and conditional skipping. Test authors only need to use test skipping

Setup the environment with hatch env create
Installed the mainline version of deadline-cloud-test-fixtures (includes aws-deadline/deadline-cloud-test-fixtures#164) into the hatch env:
```
$ hatch shell
(deadline-cloud-worker-agent) $ pip install git+https://github.com/aws-deadline/deadline-cloud-test-fixtures.git
```
Ran the end-to-end tests.
- Results pending...

Yes, updated test/e2e/README.md with updated documentation for writing os-specific tests.

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aws-deadline / deadline-cloud-worker-agent