GH-33643: [C++] Remove implicit = capture of this which is not valid in c++20

[westonpace]

• Closes: [C++] source_node.cc fails to compile on C++20 #33643

[github-actions]

• Closes: [C++] source_node.cc fails to compile on C++20 #33643

[westonpace]

@github-actions crossbow submit -e CMAKE_CXX_STANDARD=20 ubuntu-cpp

[github-actions]

No such option: -e
The Archery job run can be found at: https://github.com/apache/arrow/actions/runs/3907028385

[westonpace]

@github-actions crossbow submit ubuntu-20.04-cpp-20

[github-actions]

Unable to match any tasks for `ubuntu-20.04-cpp-20`
The Archery job run can be found at: https://github.com/apache/arrow/actions/runs/3907058236

[westonpace]

@github-actions crossbow submit test-ubuntu-20.04-cpp-20

[github-actions]

Revision: 964b96a

Submitted crossbow builds: ursacomputing/crossbow @ actions-1937fb2067

Task	Status
test-ubuntu-20.04-cpp-20

[ursabot]

Benchmark runs are scheduled for baseline = e1027dc and contender = b55dd0e. b55dd0e is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.9% ⬆️0.06%] test-mac-arm
[Finished ⬇️1.02% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.65% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] b55dd0e6 ec2-t3-xlarge-us-east-2
[Finished] b55dd0e6 test-mac-arm
[Finished] b55dd0e6 ursa-i9-9960x
[Finished] b55dd0e6 ursa-thinkcentre-m75q
[Finished] e1027dc7 ec2-t3-xlarge-us-east-2
[Finished] e1027dc7 test-mac-arm
[Finished] e1027dc7 ursa-i9-9960x
[Finished] e1027dc7 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