Add fenix nightly to integration test file paths

[jaredlockhart]

I just noticed a Fenix Nightly build update go by that didn't trigger the Fenix integration tests:

Changed files:
experimenter/tests/firefox_desktop_beta_build.env
experimenter/tests/firefox_fenix_nightly_build.env
No changes in experimenter/experimenter/targeting|experimenter/experimenter/experiments|experimenter/tests/firefox_fenix_beta_build.env|experimenter/tests/firefox_fenix_release_build.env or .circleci or root directory. Skipping tests and linting.

Let's add the Fenix nightly env file to the file paths for Fenix tests.

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