SKAFFOLD_UPDATE_CHECK should also be a global flag

[dgageot]

This specific env var was introduced before we mirrored every command line flag with env vars.
Make sure it's now available either way and properly documented.

Signed-off-by: David Gageot david@gageot.net

[codecov]

Codecov Report

Merging #4510 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4510      +/-   ##
==========================================
- Coverage   72.35%   72.35%   -0.01%     
==========================================
  Files         333      333              
  Lines       12959    12956       -3     
==========================================
- Hits         9377     9374       -3     
  Misses       2984     2984              
  Partials      598      598              

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/cmd.go	66.42% <100.00%> (+0.24%)	⬆️
pkg/skaffold/update/update.go	53.57% <100.00%> (-5.81%)	⬇️

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[dgageot]

@briandealwis could you take a look at this one. It doesn't really change anything. Just that it adds the documentation for a env var that we support.