feat: implement cross-platform support for GCB build env

[gsquared94]

Related: #6120

Description

This PR implements support for running cross-platform builds for GoogleCloudBuild build environment.

Testing instructions

1. In this branch, go to directory integration/examples/cross-platform-builds
2. Run:

   skaffold build --default-repo=gcr.io/k8s-skaffold --platform=linux/arm64 -p googleCloudBuild --cache-artifacts=false
   

   This will build the image for platform linux/arm64 on the GoogleCloudBuild build env.
3. Verify the image was built for the right target, run:

   docker run --platform=linux/arm64 gcr.io/k8s-skaffold/skaffold-example:<tag>
   

   Should produce the output:

   Hello world! Running on linux/arm64
   Hello world! Running on linux/arm64
   ...
   

[codecov]

Codecov Report

Merging #7134 (37fc75b) into main (290280e) will decrease coverage by 2.07%.
The diff coverage is 56.96%.

@@            Coverage Diff             @@
##             main    #7134      +/-   ##
==========================================
- Coverage   70.48%   68.40%   -2.08%     
==========================================
  Files         515      560      +45     
  Lines       23150    26239    +3089     
==========================================
+ Hits        16317    17950    +1633     
- Misses       5776     7055    +1279     
- Partials     1057     1234     +177     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/deploy.go	52.00% <ø> (-1.85%)	⬇️
cmd/skaffold/app/cmd/dev.go	84.61% <0.00%> (ø)
cmd/skaffold/app/cmd/render.go	36.66% <0.00%> (-4.72%)	⬇️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/fix.go	68.85% <40.00%> (-7.62%)	⬇️
cmd/skaffold/app/cmd/lint.go	42.85% <42.85%> (ø)
cmd/skaffold/app/cmd/find_configs.go	48.88% <50.00%> (+0.24%)	⬆️
cmd/skaffold/app/skaffold.go	76.19% <70.00%> (-8.43%)	⬇️
... and 215 more

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