ci: use --locked for MSRV cargo-ndk install

A transitive dependency of `cargo-ndk`, `regex-automata`,increased its
MSRV and so installing `cargo-ndk` at version 2.12.7 breaks in the MSRV
CI task unless we use the pinned `Cargo.lock` version, or upgrade our MSRV
in turn.

Author: cpu

.github/workflows/ci.yml

@@ -78,7 +78,7 @@ jobs:
 
       - name: Install cargo-ndk.
         run: |
-          cargo install cargo-ndk --version 2.12.7
+          cargo install cargo-ndk --locked --version 2.12.7
           rustup target add aarch64-linux-android
 
       - name: Clippy (${{ matrix.os }})