Make status-check flag nillable

[maggieneterval]

Fixes: N/A
Related: #4904
Merge before/after: Merge before #5706

Description
As suggested by @gsquared94 and @MarlonGamez, make the status-check flag nullable in order to distinguish among true, false, and unset values. There is no user-facing impact of this change, but it will enable us to add a new StatusCheck field in the skaffold.yaml (#5706) that, when set to false, will disable the healthcheck when the status-check flag is false or unset.

Follow-up Work (remove if N/A)
Update #5706.

[codecov]

Codecov Report

Merging #5712 (bc339e3) into master (46d1c2f) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5712      +/-   ##
==========================================
+ Coverage   70.53%   70.77%   +0.24%     
==========================================
  Files         413      413              
  Lines       15982    16004      +22     
==========================================
+ Hits        11273    11327      +54     
+ Misses       3879     3845      -34     
- Partials      830      832       +2     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/flags.go	89.02% <ø> (ø)
pkg/skaffold/config/options.go	100.00% <ø> (ø)
pkg/skaffold/config/types.go	100.00% <100.00%> (ø)
pkg/skaffold/runner/runcontext/context.go	84.34% <100.00%> (+4.88%)	⬆️
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
pkg/skaffold/event/event.go	91.96% <0.00%> (+0.84%)	⬆️
pkg/skaffold/docker/parse.go	87.14% <0.00%> (+0.95%)	⬆️
pkg/skaffold/deploy/status/status_check.go	69.69% <0.00%> (+18.93%)	⬆️

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[gsquared94]

lgtm!