`[shell,batch]_commands` can silently fail.

[UebelAndre]

Noticed on https://buildkite.com/bazel/rules-rust-rustlang/builds/8734#0188b4bf-9114-4a94-9319-62e42cc4610d for the new post commands and suspect it's the same case for startup commands. Any suggestions on how to fix?

[UebelAndre]

cc @meteorcloudy

[meteorcloudy]

Not sure, we do have fail_if_nonzero on by default, somehow the bash command exited with 0?
https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/bazelci.py#L2645