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GH-35911: [Go] Fix method CastToBytes of decimal256Traits (#35912)



### Rationale for this change

### What changes are included in this PR?

### Are these changes tested?
Yes

### Are there any user-facing changes?
Yes

* Closes: #35911

Authored-by: izveigor <izveigor@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>