Rather than hardcoding the NDK version inside a script, encode it
as a file in the filesystem, which is easier to share.
Make sure we use the desired NDK by setting environment variables.
Use `-androidabi 21`, which:
1. is what rclone did: 8390ba4ca9
2. is the minimum ABI used by probe-android: 994651be52/app/build.gradle (L10)
Part of https://github.com/ooni/probe/issues/2130
