Fix integrations-core builder $PYTHON_VERSION warnings

[Kyle-Neale]

What does this PR do?

This PR fixes integrations-core builder PYTHON_VERSION warnings

Motivation

 1 warning found (use docker --debug to expand):
 - UndefinedVar: Usage of undefined variable '$PYTHON_VERSION' (did you mean $PYTHON3_VERSION?) (line 50)

https://github.com/DataDog/integrations-core/actions/runs/12716427655/job/35450751933?pr=19372#step:10:26669

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.58%. Comparing base (fd0bf69) to head (86958d6).
Report is 12 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
presto	?
solr	?

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