Modify Travis directive restricting builds to master

[briandealwis]

The only-build-master directive prevents people from getting feedback on personal branches in forks, unless they are called master. This settings requires creating PRs for experimentation which causes noise, even if they are marked as draft.

This setting was originally brought it for #2607 to avoid merge failures from release branches. This PR instead uses a blocklist to prevent builds on release branches and version tags.

[codecov]

Codecov Report

Merging #5219 (fb42a29) into master (95092ad) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5219      +/-   ##
==========================================
+ Coverage   71.84%   71.87%   +0.02%     
==========================================
  Files         387      387              
  Lines       13974    13974              
==========================================
+ Hits        10040    10044       +4     
+ Misses       3195     3193       -2     
+ Partials      739      737       -2     

Impacted Files	Coverage Δ
pkg/skaffold/util/tar.go	56.00% <0.00%> (+5.33%)	⬆️

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[gsquared94]

What I did was to add my fork to Travis directly. I think that approach is better than modifying this repo's settings. WDYT?

[briandealwis]

I tried that, but Travis would only build if I pushed my branch to master. The description for the Building pushed branches setting notes:

Build pushed branches

If ON, builds will be run on branches that are not explicitly excluded in your .travis.yml.

I'll ping you tomorrow to compare settings tomorrow.