addBackcompatIndexes.py on 9x can't handle 8x indices [LUCENE-10625]

When going through the release wizard for 8x, steps that involve running `python3 -u dev-tools/scripts/addBackcompatIndexes.py` will fail because the python script will try to run gradle commands inside of the 8x release to generate the back combat indices.

We could maybe be more robust in detecting that the index already exists in the temp directory? I'm not actually sure how to fix this, and it might not matter much as I don't know how many additional 8.11 releases we will have, but reporting for posterity.

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Migrated from [LUCENE-10625](https://issues.apache.org/jira/browse/LUCENE-10625) by Mike Drob (@madrob)

Start with dev-tools/scripts/addBackcompatIndexes.py and the release-wizard steps that run it. Trace why the script invokes Gradle commands inside the 8x release and inspect how it handles indices in the temporary directory. Done means the 9x script can process 8x indices without failing.

Written by the indexing model from the issue text.