cbaeef2879d4c4 recently refactored the evergreen configuration file. This subtly introduced some
breakage, because the `env` and `silent` options are not command options, but rather need to
be nested under the `params` key.

I've intentionally left the `silent:` option disabled as this would hide genuinely valuable
debug output otherwise.

This might be a good point to reconsider fixing the evergreen validation warnings that are
displayed in CI; `evergreen validate .evergreen.yml` did catch these issues, but since there
are a large number of existing warnings (around dependencies between tasks), these specific
warnings were drowned out/not visible in the evergreen UI.