skaffold diagnose to not initialize runconfig for yaml only flag

[tejal29]

Fixes #5701

In this PR the code is refactored to make sure we don't create a skaffold RunContext object when skaffold diagnose is run with --yaml-only flag.

[codecov]

Codecov Report

Merging #5762 (50836aa) into master (21e3c95) will decrease coverage by 0.19%.
The diff coverage is 33.69%.

@@            Coverage Diff             @@
##           master    #5762      +/-   ##
==========================================
- Coverage   70.89%   70.70%   -0.20%     
==========================================
  Files         421      427       +6     
  Lines       16091    16181      +90     
==========================================
+ Hits        11408    11441      +33     
- Misses       3849     3899      +50     
- Partials      834      841       +7     

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/v1/config.go	58.82% <ø> (ø)
pkg/skaffold/schema/v2beta14/upgrade.go	100.00% <ø> (ø)
pkg/skaffold/schema/versions.go	82.41% <ø> (ø)
pkg/skaffold/schema/v2beta15/config.go	20.58% <20.58%> (ø)
cmd/skaffold/app/cmd/diagnose.go	63.33% <50.00%> (+30.00%)	⬆️
cmd/skaffold/app/cmd/runner.go	58.49% <100.00%> (-1.51%)	⬇️
pkg/skaffold/runner/runcontext/context.go	85.12% <100.00%> (+0.37%)	⬆️
pkg/skaffold/schema/v2beta15/upgrade.go	100.00% <100.00%> (ø)
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
... and 8 more

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