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Fix unnecessary rebuilds (#1258)

This PR addresses an issue in the new `contracts` crate when it was being rebuilt when it wasn't necessary. Since this is use by almost all other crates in this project, it would lead to unnecessarily long build times.

### Test Plan

Observe it would rebuild `contracts` and dependent crates unnecessarily on the base branch:
```
$ git checkout npm-contracts
$ cargo build --workspace --all-features
...
$ touch e2e/src/lib.rs && cargo build --workspace --all-features # BOO!
   Compiling contracts v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/contracts)
   Compiling core v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/core)
   Compiling e2e v1.0.0 (/var/home/nlordell/Developer/gnosis/dex-services/e2e)
   Compiling driver v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/driver)
   Compiling price-estimator v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/price-estimator)
    Finished dev [unoptimized + debuginfo] target(s) in 42.31s
```

Now check the fix:
```
$ git checkout fix-cargo-rebuild-contracts
$ cargo build --workspace --all-features
...
$ touch e2e/src/lib.rs && cargo build --workspace --all-features # YAY!
   Compiling e2e v1.0.0 (/var/home/nlordell/Developer/gnosis/dex-services/e2e)
    Finished dev [unoptimized + debuginfo] target(s) in 8.67s
```

And verify that deployments and modifying the `target/deploy/*.addr` files still correctly trigger rebuilds:
```
$ (cd contracts; cargo run --bin deploy --all-features) && cargo build --workspace --all-features
   Compiling contracts v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/contracts)
   Compiling core v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/core)
   Compiling e2e v1.0.0 (/var/home/nlordell/Developer/gnosis/dex-services/e2e)
   Compiling driver v0.1.0 (/var/home/nlordell/Developer/gnosis/dex-services/driver)
   Compiling price-estimator v0.1.0 (/var/home/nlordell/... (continued)

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