Stop excluding /api/client/BUILD in check_format

We exclude `api/client/BUILD` in `check_format.sh`, because the build insists on adding

```
load("bazel:envoy_build_system.bzl", "envoy_package")
```
For this to work for `Nighthawk`, this should look like:

```
load("@envoy//bazel:envoy_build_system.bzl", "envoy_package")
```

So either having a way to pull that off, or a way to make a more narrow exclusion instead of excluding the whole file from `check_format` would be desirable.