[9.20] fix: dev: Take dns_dtenv_t reference before an async function call

A 'dns_dtenv_t' pointer is passed to an async function without taking
a reference first, which can potentially cause a use-after-free error.
Take a reference, then detach in the async function.

Closes #5820

Backport of MR !11705

Merge branch 'backport-5820-dns_dtenv-reference-bug-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11714