Enable MTU path discovery on Linux and setup registry-mirrors for kind

[briandealwis]

Third attempt is the charm!

See #5208 (comment) and #5221 for context.

[codecov]

Codecov Report

Merging #5237 (b3ac7ca) into master (98faaaa) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5237      +/-   ##
==========================================
- Coverage   71.87%   71.87%   -0.01%     
==========================================
  Files         387      388       +1     
  Lines       13974    14036      +62     
==========================================
+ Hits        10044    10088      +44     
- Misses       3193     3205      +12     
- Partials      737      743       +6     

Impacted Files	Coverage Δ
pkg/skaffold/util/util.go	83.19% <0.00%> (-0.15%)	⬇️
pkg/skaffold/docker/docker_init.go	100.00% <0.00%> (ø)
pkg/skaffold/initializer/build/util.go	100.00% <0.00%> (ø)
pkg/skaffold/trigger/fsnotify/trigger.go	71.05% <0.00%> (ø)
pkg/skaffold/initializer/build/builders.go	70.00% <0.00%> (ø)
pkg/skaffold/initializer/analyze/builder.go	100.00% <0.00%> (ø)
pkg/skaffold/trigger/fsnotify/workdir.go
pkg/skaffold/tags/paths.go	65.00% <0.00%> (ø)
pkg/skaffold/util/workdir.go	20.00% <0.00%> (ø)
pkg/skaffold/docker/image.go	79.53% <0.00%> (+1.39%)	⬆️

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[briandealwis]

Merging: I've re-run the build several times and it's come up green every time.