Use `set -gx` instead of `setenv` for fish shell.

Fix test "activate if the first virtualenv is a virtualenv": it was asserting
the output was of bash form, but not specifying that in the test — it now does.

Fixes https://github.com/pyenv/pyenv-virtualenv/issues/216.

* https://github.com/yyuu/pyenv/pull/615
* https://github.com/fish-shell/fish-shell/issues/310

It was previously evaluated when 'pyenv virtualenv-init' is run.

`pyenv virtualenv-init` is just an option. Always showing message about
it would be too much to do.

```sh
% pyenv versions
  system
* miniconda3-3.16.0 (set by PYENV_VERSION environment variable)
  miniconda3-3.16.0/envs/venv
% pyenv activate venv
pyenv-virtualenv: deactivate miniconda3-3.16.0
pyenv-virtualenv: activate miniconda3-3.16.0/envs/venv
```

conda's environment might not have `python` executable. If the prefix
doesn't contain `python` in it, `pyenv-which` might be ran into infinite
loop if some of `which` hooks invoke `pyenv-virtualenv-prefix`.

This means that pyenv-virtualenv will not allow manual deactivation of
virtualenv if pyenv-virtualenv-init is enabled. This must be acceptable
since the activation of virtualenv is just setting of environment
variables in current implementation.

The `activate` script does't export the variables of `_OLD_VIRTUAL_*`,
most the deactivation code needed to be moved to `sh-deactivate`.