MINOR: [CI][C++] Fix Docker caching on JNI builds

[pitrou]

Rationale for this change

Docker caching didn't work on JNI builds as the Docker volume storage didn't match the cached directory.

What changes are included in this PR?

Move the DOCKER_VOLUME_PREFIX definition at the C++ Extra workflow top level so that it also affects Docker-based JNI jobs.

Are these changes tested?

Yes, by existing CI runs.

Before:

• JNI ubuntu-latest amd64: 51 minutes
• JNI ubuntu-24.04-arm arm64/v8: 34 minutes

After:

• JNI ubuntu-latest amd64: 14 minutes
• JNI ubuntu-24.04-arm arm64/v8: 10 minutes

Are there any user-facing changes?

No.

[github-actions]

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See also:

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[pitrou]

@kou Is this ok?

Note that the builds themselves will be fixed by #50741