Ensure that consumers of Executors#startBlockingBuild stop the blocking build

[basil]

#8157 addresses the surface level of the problem in ExecutorTest but does not address the root cause of the problem, which affects any test that calls ExecutorTest#startBlockingBuild.

Testing done

N/A

Proposed changelog entries

• Entry 1: Issue, human-readable text
• […]

Proposed upgrade guidelines

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1. The Jira issue, if it exists, is well-described.
2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
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@mention

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[jglick]

Seems cleaner.

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!