rbe_configs_gen produces wrongly formatted Starlark code (flagged by buildifier)

I generate RBE config using this command:

```
$ ./rbe_configs_gen \
--bazel_version=4.1.0 \
--toolchain_container=l.gcr.io/google/rbe-ubuntu18-04:latest \
--output_src_root=/home/davido/projects/gerrit \
--output_config_path=tools/rbe \
--exec_os=linux \
--target_os=linux
```

and running the latest `buildifier` utility on generated code produces this diff: [1].

Used buildifier version:

```
$ buildifier -version
buildifier version: 4.0.0
buildifier scm revision: 840218fa3afc7e7145c1eeb3bfeb612c497e67f7
```

[1] http://paste.openstack.org/show/805685