Add `FORCE_COLOR` and `NO_COLOR` variables to the `isolate()` fixture
to ensure that these two variables do not affect internal test output.
This fixes the following two test failures when pytest is called with
`NO_COLOR` set:

```
FAILED tests/unit/test_exceptions.py::TestDiagnosticPipErrorPresentation_ASCII::test_complete_color - AssertionError: assert '\x1b[1merror...ing harder.\n' == '\x1b[1;31mer...ing harder.\n'
FAILED tests/unit/test_exceptions.py::TestDiagnosticPipErrorPresentation_Unicode::test_complete_color - AssertionError: assert '\x1b[1merror...ing harder.\n' == '\x1b[1;31mer...ing harder.\n'
```

This reverts commit 1b33f4b.  Now
the test suite strips `FORCE_COLOR` internally, so it should be fine
to run with colors enabled for nox.

Lower the scope of `simple_script` fixture used in `test_inspect`
from `session` to (implicit) `function`, to make it match the `isolate`
fixture and therefore make the fixture run in isolation.
Per the documentation [1], pytest initializes higher-scope fixtures
first, and lower-scope fixtures cannot be used in them.  Since within
the same scope, autouse fixtures are always loaded before regular
fixtures, no further adjustment is necessary.  This should have no ill
side effects, as the fixture is used by a single test only.

[1] https://docs.pytest.org/en/stable/reference/fixtures.html#fixture-instantiation-order