* Skip verifying checksums for already fetched packages.
Needed for two-staged building in Nix:
1. Build a fixed derivation with `bazel fetch` (filtered out of non-reproducable bits).
2. Build an actual derivation which uses fetched dependencies (skipping
checksums needed here because they depend on the build directory).
* Don't clean environment variables for children processes.
Needed for Nix compiler wrappers.
