change nginx process pgid

[oilbeater]

What this PR does / why we need it:
put restarted nginx process in another process group, just like the normal nginx start did in https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/controller/nginx.go#L289

[codecov-io]

Codecov Report

Merging #2181 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2181      +/-   ##
==========================================
- Coverage   36.45%   36.43%   -0.03%     
==========================================
  Files          69       69              
  Lines        4861     4864       +3     
==========================================
  Hits         1772     1772              
- Misses       2819     2822       +3     
  Partials      270      270

Impacted Files	Coverage Δ
internal/ingress/controller/nginx.go	3.92% <0%> (-0.04%)	⬇️

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[aledbf]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, oilbeater

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[aledbf]

@oilbeater thanks!