roachtest: add v10.TestBigColumn to gopg blocklist

[abhishek-dalbanjan]

This PR adds the unexpectedly failing test v10.TestBigColumn to the gopgBlockList inside pkg/cmd/roachtest/tests/gopg_blocklist.go.

The test was reported to be failing unexpectedly on the release branch, blocking releases. By adding it to the known failures blocklist, we allow CI to pass and unblock the release pipeline while the root cause is investigated.

Fixes #172196

Release note: None

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