Fix master branch build

[Sovietaced]

Why are the changes needed?

Master builds are failing.

What changes were proposed in this pull request?

1. Pin buf
2. Use a minio image that can be pulled
3. Improve test assertions

How was this patch tested?

CI

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• changed: For changes in existing functionality.
• deprecated: For soon-to-be-removed features.
• removed: For features being removed.
• fixed: For any bug fixed.
• security: In case of vulnerabilities

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• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.29%. Comparing base (e2620f7) to head (5ecb68a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8065      +/-   ##
==========================================
- Coverage   57.29%   57.29%   -0.01%     
==========================================
  Files         931      931              
  Lines       58316    58316              
==========================================
- Hits        33413    33412       -1     
- Misses      21844    21845       +1     
  Partials     3059     3059              

Flag	Coverage Δ
unittests-datacatalog	53.62% <ø> (ø)
unittests-flyteadmin	53.24% <ø> (ø)
unittests-flytecopilot	48.05% <ø> (ø)
unittests-flytectl	64.11% <ø> (ø)
unittests-flyteidl	76.63% <ø> (ø)
unittests-flyteplugins	60.45% <ø> (ø)
unittests-flytepropeller	53.84% <ø> (ø)
unittests-flytestdlib	64.39% <ø> (-0.03%)	⬇️

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