## Issue Addressed
#3369
## Proposed Changes
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.
Philip White
cf3bcca969