Use --locked in docs for how to install libcnb-cargo (#734)

By default `cargo install` will not use any lockfile in the published
crate, and instead will install the latest in-range version of any
transitive dependencies.

By using `--locked` the lockfile will be used, ensuring a more
deterministic installation.

Note: Whilst this repo intentionally doesn't commit a lockfile to
Git (since this is a library crate), Cargo will include one in the
crate uploaded to crates.io at time of publish, so `--locked` will
still work.

See:
https://doc.rust-lang.org/cargo/commands/cargo-install.html#dealing-with-the-lockfile

README.md

@@ -31,7 +31,7 @@ Start by installing [libcnb-cargo](https://crates.io/crates/libcnb-cargo), which
 that we will use later to package our buildpack:
 
 ```shell
-cargo install libcnb-cargo
+cargo install --locked libcnb-cargo
 ```
 
 #### Cross-compilation prerequisites

libcnb-cargo/README.md

@@ -5,7 +5,7 @@ A Cargo command for managing buildpacks written with [libcnb.rs](https://github.
 ## Installation
 
 ```shell
-cargo install libcnb-cargo
+cargo install --locked libcnb-cargo
 ```
 
 ## Usage