BazelCI Failing for bazel-toolchains with release Bazel

[c-parsons]

Failing on RBE with 16.04/

Example buildkite failure:
https://buildkite.com/bazel/bazel-toolchains/builds/22557#0a001b16-56d1-47e8-a9fc-6476b826408f

Seems to have been failing since Dec 17.

Executing tests from //tests/rbe_repo:rbe_autoconf_custom_toolchain_config_suite_spec_export_test
-----------------------------------------------------------------------------
File external/rbe_autoconf_custom_toolchain_config_suite_spec_export/test/rbe_autoconf_root/rbe-test-output/config/rbe_autoconf_custom_toolchain_config_suite_spec_export/test_config_dir/bazel_3.7.1/cc/test.BUILD does not exist.

[Wyverald]

This started failing again: https://buildkite.com/bazel/bazel-toolchains/builds/23838#58641906-6b5e-4656-a812-893c5982d2cf

It was passing from 2021-01-07 to 2021-01-21. Could very likely be due to new Bazel releases? (4.0 was just released yesterday.)

[Wyverald]

See also bazelbuild/bazel#12455 (comment)

[meteorcloudy]

Fixed by #925