fix: use locked Cargo.lock for installation

This PR adds `--locked` to the installation command
`cargo instal --locked --path .`. This flag locks Cargo.lock.

Without this flag, rust will actually update the dependencies prior to
installing the binaries. If someone upstream introduces a backwards-
incompatible error, we are stuck with the compilation errors.

Co-authored-by: Ferdinand Sauer <ferdinand@neptune.cash>

README.md

@@ -15,7 +15,7 @@ Neptune-core is the reference implementation for the [Neptune](https://neptune.c
  - Enter the repository: `cd neptune-core`
  - Checkout the release branch `git checkout release`. (Alternatively, for the *unstable development* branch, skip this step.)
 
- - Build for release and put the binaries in your local path (`~/.cargo/bin/`): `cargo install --path .` (needs at least 3 GB of RAM and a few minutes)
+ - Build for release and put the binaries in your local path (`~/.cargo/bin/`): `cargo install --locked --path .` (needs at least 3 GB of RAM and a few minutes)
 
 ### Windows
 
@@ -28,7 +28,7 @@ With a functioning version of cargo, compilation on Windows should just work out
 - Enter the repository: `cd neptune-core`
 - Checkout the release branch `git checkout release`. (Alternatively, for the *unstable development* branch, skip this step.)
 
-- Run `cargo install --path .`
+- Run `cargo install --locked --path .`
 
 
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