Add SWIFT_AVOID_WARNING_USING_OLD_DRIVER to environment
Add SWIFT_AVOID_WARNING_USING_OLD_DRIVER to environment

Since
https://github.com/apple/swift/commit/e24eef4b35290135634c11a9a83e652de1e4919d
Swift now emits a warning about the old driver being deprecated. If you
enable the old driver flag we can assume you are doing that
intentionally for now and know that isn't a forever solution.

bazelbuild/rules_swift,bazelbuild/rules_swift