readme: add --locked to cargo install step

Our installation instructions don't currently work with Rust < 1.64
because `clap` updated their MSRV to 1.64, and `cargo install` without
`--locked` bypasses `Cargo.lock` and selects the version of `clap`
that needs Rust 1.64.

README.md

@@ -164,7 +164,7 @@ sudo apt-get install libssl-dev openssl pkg-config
 
 Now run:
 ```shell script
-cargo install --git https://github.com/martinvonz/jj.git --bin jj jujutsu
+cargo install --git https://github.com/martinvonz/jj.git --locked --bin jj jujutsu
 ```
 
 
@@ -226,15 +226,15 @@ export PKG_CONFIG_PATH="$(brew --prefix)/opt/openssl@3/lib/pkgconfig"
 
 Now run:
 ```shell script
-cargo install --git https://github.com/martinvonz/jj.git --bin jj jujutsu
+cargo install --git https://github.com/martinvonz/jj.git --locked --bin jj jujutsu
 ```
 
 
 ### Windows
 
 Run:
 ```shell script
-cargo install --git https://github.com/martinvonz/jj.git --bin jj jujutsu --features vendored-openssl
+cargo install --git https://github.com/martinvonz/jj.git --locked --bin jj jujutsu --features vendored-openssl
 ```