Allow setting web3signer version through environment

The goal is to make it possible to build Lighthouse without network access,
so builds can be reproducible.

This parallels the existing functionality in `common/deposit_contract/build.rs`,
which allows specifying a filename through the environment to avoid downloading
it. In this case, by specifying the version and making it available on the
filesystem, the existing logic will avoid a network download.

Author: philipmw

.gitignore

@@ -9,3 +9,6 @@ perf.data*
 /bin
 genesis.ssz
 /clippy.toml
+
+# IntelliJ
+/*.iml

testing/web3signer_tests/build.rs

@@ -10,7 +10,9 @@ use zip::ZipArchive;
 
 /// Use `None` to download the latest Github release.
 /// Use `Some("21.8.1")` to download a specific version.
-const FIXED_VERSION_STRING: Option<&str> = None;
+const FIXED_VERSION_STRING: Option<String> =
+    env::var("LIGHTHOUSE_WEB3SIGNER_VERSION")
+    .ok();
 
 #[tokio::main]
 async fn main() {