RBE: Update toolchain with bazel-toolchains 5.1.2 release

RBE: Update toolchain with bazel-toolchains 5.1.2 release

Due to this platform style migration: [1] the RBE toolchain needs to be
updated to use the latest rbe_config_gen from bazel-toolchains (at least
version 5.1.2 so that it contains: [2]).

This change makes RBE build forwards compatible so that Bazel could be
updated to the upcoming major 6.0 release.

[1] https://github.com/bazelbuild/bazel/issues/8622
[2] https://github.com/bazelbuild/bazel-toolchains/commit/8c717f8258cd5f6c7a45b97d974292755852b658

Change-Id: Ia3a108fa05b1d52cd2018ba901ffd4c9e55ae8ab