fix(module:steps): fix steps state change error under onpus strategy

[CoderPoet]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

when we use steps under onpush stategy , modify the nzCurrent can correct diplay step status.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[codecov]

Codecov Report

Merging #3061 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3061   +/-   ##
=======================================
  Coverage   97.32%   97.32%           
=======================================
  Files         549      549           
  Lines       11490    11490           
  Branches      815      815           
=======================================
  Hits        11183    11183           
  Misses        197      197           
  Partials      110      110

Impacted Files	Coverage Δ
components/steps/nz-steps.component.ts	100% <100%> (ø)	⬆️
components/steps/nz-step.component.ts	97.14% <100%> (ø)	⬆️

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[wzhudev]

LGTM