Read Capybara javascript_driver from CAPYBARA_JS_DRIVER

[wuservices]

Summary

Updates env variable config to be aligned with similar config in Solidus Starter Frontend.

This is helpful because setting CAPYBARA_JS_DRIVER=selenium_chrome_headless_docker_friendly will allow tests to run with a Docker-friendly Chrome config, while keeping the default rack_test driver for non-JS tests.

Without this change, CAPYBARA_DRIVER would have to be set to use a custom driver for JS tests, but then it would conflict with the CAPYBARA_DRIVER env variable for non-JS tests.

There are no other instances of CAPYBARA_DRIVER in this repo, and no related CI, so there's nothing else to update. However, this could be a breaking change for anybody depending on the existing env variable.

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