CI: Use a mode=min in the builder cache

[tenzen-y]

What this PR does / why we need it:
I replaced cache-to mode with min (default value) to avoid the following timeout errors:

#7 writing layer sha256:202111e1e67fd289b555ea0a5f198b9919220afe610c9e070268fdd75c3dd5e2 600.1s done
#7 ERROR: error writing layer blob: maximum timeout reached
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 > exporting to GitHub Actions Cache:
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WARNING: No output specified with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load
ERROR: failed to solve: error writing layer blob: maximum timeout reached
Error: buildx failed with: ERROR: failed to solve: error writing layer blob: maximum timeout reached

https://github.com/kubeflow/training-operator/actions/runs/8635377472/job/23674563980?pr=2051#step:5:1294

xref: docker/buildx#841

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #

Checklist:

• Docs included if any changes are user facing

[tenzen-y]

cc: @johnugeorge

[johnugeorge]

/lgtm

Thanks @tenzen-y

[coveralls]

Pull Request Test Coverage Report for Build 8636359575

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 35.227%

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Totals
Change from base Build 8635271448:	0.06%
Covered Lines:	4341
Relevant Lines:	12323

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💛 - Coveralls

[terrytangyuan]

Thanks!

/lgtm

[google-oss-prow]

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