Simplify BuildSubtaskEvent ID setting/handling

[MarlonGamez]

Related: #5368 #5370

Description
Changes the ID field BuildSubtaskEvents to be the artifact name, rather than an integer corresponding to the artifact name. This makes the code more simple and easier, especially when it comes to tying to the skaffoldLogEvent output

[codecov]

Codecov Report

Merging #6167 (3f58723) into master (a4cf386) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6167   +/-   ##
=======================================
  Coverage   69.36%   69.36%           
=======================================
  Files         481      481           
  Lines       18480    18475    -5     
=======================================
- Hits        12819    12816    -3     
+ Misses       4706     4705    -1     
+ Partials      955      954    -1     

Impacted Files	Coverage Δ
pkg/skaffold/runner/build.go	72.82% <ø> (-0.30%)	⬇️
pkg/skaffold/build/scheduler.go	92.18% <100.00%> (ø)
pkg/skaffold/event/v2/build.go	100.00% <100.00%> (+7.69%)	⬆️
pkg/skaffold/server/server.go	41.96% <0.00%> (ø)

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