rbe_configs_gen: confusing error when custom container contains .bazelrc

*Disclaimer: I understand this may be an invalid use case and I am using the tool tool incorrectly 🙃*

## Background:

My team needs to produce native CPP build for CentOS and CentOS 7 has an older version of glibc so we are using a CentOS-based custom container[1].

I thought I would be clever and define a custom platform within my sandbox container and included a `build --host_platform //toolchain:centos` in the bazelrc of our container allowing us to use constraints to prevent someone from accidentally building e.g. an RPM on Ubuntu.

## Issue

Fast forward to an investigation into using RBE and I got a confusing error when running rbe_configs_gen

```
ERROR: While resolving toolchains for target @local_config_cc//:cc-compiler-k8: com.google.devtools.build.lib.packages.BuildFileNotFoundException: no such package 'toolchain': BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.
- /workdir/cpp_configs_project/toolchain
ERROR: Analysis of target '@local_config_cc//:cc-compiler-k8' failed; build aborted: com.google.devtools.build.lib.packages.BuildFileNotFoundException: no such package 'toolchain': BUILD file not found in any of the following directories. Add a BUILD file to a directory to mark it as a package.
- /workdir/cpp_configs_project/toolchain
```

`@local_config_cc//` wha? is Bazel somehow broken?

I now now realize this was due to Bazel reading the `.bazelrc` defined in the container and this is admittedly self-inflicted but it took me quite a while to figure this out. Removing the .bazelrc file from our container solves the problem.

## Suggested Solutions

1) Do nothing, hopefully anyone who runs into this issue finds this issue by the keywords I included in this issue

1) add `--bazelrc=/dev/null` to the bazel commands run by rbe_configs_gen

I think doing nothing is a perfectly acceptable solution, my main reason for raising this issue is making it searchable in case anyone makes the same mistake in the future.

---
[1] Maybe we would be better served by extending an existing ubuntu rbe base image and installing the desired version of GCC *but* for now we are exploring making a custom container based on CentOS to try to closely match the target environment.

Start at the rbe_configs_gen entry point and inspect the Bazel commands it runs, focusing on how a custom container's .bazelrc is picked up. Compare the proposed --bazelrc=/dev/null behavior with the current commands; done should mean the tool no longer produces a misleading toolchain error from the container's Bazel configuration.

Written by the indexing model from the issue text.