Opt-out of PATH changes

Bazelisk changes the PATH variable, which has deleterious consequences for nix-based builds, and it seems like a number of other scenarios. There should be a way to ensure that all environment variables are passed through faithfully without modification to the underlying bazel executable.

Related:
- https://github.com/bazelbuild/bazelisk/issues/471
- https://github.com/bazelbuild/bazelisk/issues/301

No implementation files or tests are named. Start by tracing Bazelisk's environment handling and review the related issues for intended behavior; done means providing an opt-out that passes environment variables through unchanged, including PATH, to the underlying Bazel executable.

Written by the indexing model from the issue text.