An error occurred during the fetch of repository 'rbe_default': key "mac os x" not found in dictionary

Hey folks,

I'm currently trying to run the examples in this repository. For context, I have a remote execution server running BuildGrid on an Amazon Linux image. I am using `bazel_toolchains` version 3.3.1.

After copying the example WORKSPACE and .bazelrc, I'm receiving this error message:

```
$ bazel clean --expunge && bazel build --config=remote //...
INFO: Starting clean.
Starting local Bazel server and connecting to it...
INFO: Invocation ID: b3a3aa84-307a-4642-91f7-33d0ae5bea1a
INFO: Options provided by the client:
Inherited 'common' options: --isatty=1 --terminal_columns=90
INFO: Found applicable config definition common:remote in file /Users/davidlee/dev/examples/java-maven/.bazelrc: --spawn_strategy=remote --strategy=Javac=remote --strategy=Closure=remote --strategy=Genrule=remote --define=EXECUTOR=remote --remote_executor=grpc://ec2-3-101-14-83.us-west-1.compute.amazonaws.com:50051 --remote_instance_name=main --announce_rc --crosstool_top=@rbe_default//cc:toolchain --host_javabase=@rbe_default//java:jdk --javabase=@rbe_default//java:jdk --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_hostjdk8 --java_toolchain=@bazel_tools//tools/jdk:toolchain_hostjdk8 --extra_execution_platforms=@rbe_default//config:platform --host_platform=@rbe_default//config:platform --platforms=@rbe_default//config:platform --extra_toolchains=@rbe_default//config:cc-toolchain
INFO: Call stack for the definition of repository 'rbe_default' which is a _rbe_autoconfig (rule definition at /private/var/tmp/_bazel_davidlee/69e0bb4508c054bbdc8964a6633bb9ad/external/bazel_toolchains/rules/rbe_repo.bzl:650:19):
-
- /private/var/tmp/_bazel_davidlee/69e0bb4508c054bbdc8964a6633bb9ad/external/bazel_toolchains/rules/rbe_repo.bzl:1170:5
- /Users/davidlee/dev/examples/java-maven/WORKSPACE:41:1
DEBUG: /private/var/tmp/_bazel_davidlee/69e0bb4508c054bbdc8964a6633bb9ad/external/bazel_toolchains/rules/rbe_repo.bzl:491:5: Bazel 3.0.0 is used in rbe_default.
ERROR: An error occurred during the fetch of repository 'rbe_default':
key "mac os x" not found in dictionary
ERROR: While resolving toolchains for target //:java-maven: com.google.devtools.build.lib.packages.RepositoryFetchException: no such package '@rbe_default//config': key "mac os x" not found in dictionary
ERROR: Analysis of target '//:java-maven' failed; build aborted: com.google.devtools.build.lib.packages.RepositoryFetchException: no such package '@rbe_default//config': key "mac os x" not found in dictionary
INFO: Elapsed time: 2.919s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (2 packages loaded, 2 targets configured)
```

Has anyone else run into this issue before? If so, how did you resolve this?