Add awk as a default tool for Bazel shell commands.

Apparently
https://github.com/gflags/gflags/blob/e292e0452fcfd5a8ae055b59052fc041cbab4abf/bazel/gflags.bzl#L8
assumes it should be accessible. Normally we could ask them to fix, but
I would expect awk to be a commonly assumed.

The rough search
https://github.com/search?q=filename%3ABUILD+genrule+awk&type=Code
brings ~1K hits.