GH-35965: [Go] Fix Decimal256DictionaryBuilder

[izveigor]

Rationale for this change

Decimal256DictionaryBuilder cannot append decimal256 value and prefers decimal128 instead.

cannot use v (variable of type decimal256.Num) as decimal128.Num value in argument to builder.Append

What changes are included in this PR?

Are these changes tested?

Yes

Are there any user-facing changes?

Yes

• Closes: Decimal256DictionaryBuilder does not work correctly #35965

[github-actions]

⚠️ GitHub issue #35965 has been automatically assigned in GitHub to PR creator.

[zeroshade]

Thanks for catching this oversight!

[ursabot]

Benchmark runs are scheduled for baseline = 9630258 and contender = cd7f431. cd7f431 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Failed] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.62% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.33% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.24% ⬆️0.06%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] cd7f431d ec2-t3-xlarge-us-east-2
[Finished] cd7f431d test-mac-arm
[Finished] cd7f431d ursa-i9-9960x
[Finished] cd7f431d ursa-thinkcentre-m75q
[Failed] 96302584 ec2-t3-xlarge-us-east-2
[Finished] 96302584 test-mac-arm
[Finished] 96302584 ursa-i9-9960x
[Finished] 96302584 ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java