This was initially added so that the __NVIM_DETACH environment variable
can be added to the jobs started in this test. That environment variable
is no longer needed, and there is also vim.tbl_extend() that can be used
to add an environment variable to a job anyway.
Also, make a shallow copy of opts.env in setup_child_nvim(), as mutating
the opts.env passed in may mask problems in other tests.
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