fix: warn instead of fail for multi-arch builds

[gsquared94]

Fixes: #7132
Related: 6120

Description

For mixed platform kubernetes clusters, skaffold was attempting to build multi-arch images. Since this feature isn't implemented yet, it was throwing an error for docker builder. This effectively breaks backwards compatibility for users of previous skaffold versions that were using mixed platform clusters.

This PR reduces these errors to warning messages asking the user to explicitly set the target platform, but continuing on with the run as before.
User facing changes (remove if N/A)

Get a warning message like:

Starting build...
Building [skaffold-example]...
Target platforms: [linux/amd64,linux/arm64]
WARN[0001] multiple target platforms "linux/amd64,linux/arm64" found for artifact "skaffold-example". Skaffold doesn't yet support multi-platform builds for the docker builder. Consider specifying a single target platform explicitly. See https://skaffold.dev/docs/pipeline-stages/builders/#cross-platform-build-support  subtask=skaffold-example task=Build
[+] Building 0.1s (11/11) FINISHED
...                                                                                            
Starting deploy...
 - pod/getting-started created
Waiting for deployments to stabilize...
 - pods: creating container getting-started
    - pod/getting-started: creating container getting-started
 - pods is ready.
Deployments stabilized in 34.975 seconds

[codecov]

Codecov Report

Merging #7145 (ce51008) into main (290280e) will decrease coverage by 2.09%.
The diff coverage is 56.96%.

@@            Coverage Diff             @@
##             main    #7145      +/-   ##
==========================================
- Coverage   70.48%   68.39%   -2.10%     
==========================================
  Files         515      560      +45     
  Lines       23150    26256    +3106     
==========================================
+ Hits        16317    17957    +1640     
- Misses       5776     7060    +1284     
- Partials     1057     1239     +182     

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 216 more

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[tejal29]

Can you add a debug statement here indicating we set Docker_Buildkit=1 and why.