add ndk r12b to any buildjni build that doesn't have an ndk

r12b has been forever the default. Let's explicitly request that version
where we know we need it.

Note; there's other builds that still implicitly rely on that version
that don't use the buildjni flag. We don't really have a good way of
finding those, except manually.
1 job for master in 5 minutes and 55 seconds (queued for 1 second)