don't install cargo-ndk in ./build-ndk.sh; pin its version in CI
- Currently ./build-ndk.sh runs
cargo install cargo-ndk
on every invocation, which is a bit silly -- and, more importantly, breaks now that the latest version ofcargo-ndk
is 3.0.0, which doesn't support older Rust versions. - Instead, install version 2.12.6 explicitly in the gitlab CI, and don't include this command in the shell script.
- We might decide to change this in future (#36 (closed)), but it's nice to explicitly pin it anyhow.