fix: nil: ignore hardening flags on plain builds

The 'plain' optimization level doesn't add any flags and gives the
control to the packager. Similarly, avoid any hardening flags in this
level.

Necessary flags such as `-fno-delete-null-pointer-checks` and
`-fno-strict-aliasing` are still included.

Merge branch 'aydin/plain-build' into 'main'

See merge request isc-projects/bind9!10673