feat(ops/buildkite): Support "soft failures" r/5227

Example Usage:

```nix
{ pkgs, ... }:

(pkgs.writeText "foo" "bar").overrideAttrs (_: {
  meta.ci.extraSteps.bar = {
    label = "testing soft_fail";
    softFail = [
      { exit_status = 253; }
      { exit_status = 251; }
    ];
    command = pkgs.writeShellScript "fail" ''
      # Trigger a soft failure by exiting 253 or 251
      exit 253
    '';
  };
})
```

Buildkite Documentation: https://buildkite.com/changelog/56-command-steps-can-now-be-made-to-soft-fail

Change-Id: Idfce15a8d9cde568aa24ebd03956ada399bba5d2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7076
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

0 files changed, 0 insertions, 0 deletions