Wins

This is a decent improvement to our local development cycle.
Previously running cargo test after modifying a test took 5.5s to build on my machine, with this PR it takes 3s to build.
I expect the improvement to be even more on machines with lower core counts than mine.

Investigation

By looking at cargo test --no-run --timings before and after the PR we can see what changed:

Before:

After:

The build script i.e. build.rs was rerunning and this was forcing all of shotover-proxy to be rebuilt and any example binaries.
Afterwards only the test binary that we modified is being rebuilt.

Solution

The problem with our current setup is that unless instructed otherwise cargo will rerun the build.rs everytime a single file is changed in the package.
The shotover-proxy/build.rs is no longer needed, I assume the user of the env var it emits was moved into test-helpers or tokio-bin-process, so I just removed it.
The other remaining build.rs files have been changed to declare the env var that they take in so that cargo doesnt default to rerunning every time.
see: https://doc.rust-lang.org/cargo/reference/build-scripts.html#change-detection