This should have been done in commit 18ce3a4a, which added that parameter
to ExplainOneQuery, but it was overlooked.  This makes it impossible for
a user of the hook to pass the queryEnv down to ExplainOnePlan.

It's too late to change this API in v10, I suppose, but fortunately
passing NULL to ExplainOnePlan will work in nearly all interesting
cases in v10.  That might not be true forever, so we'd better fix it.

Tatsuro Yamada, reviewed by Thomas Munro

Discussion: https://postgr.es/m/890e8dd9-c1c7-a422-6892-874f5eaee048@lab.ntt.co.jp