gpMgmt: skip downloading existing Python dependency tarballs

[yjhjstz]

Fixes #ISSUE_Number

What does this PR do?

Type of Change

• Bug fix (non-breaking change)
• New feature (non-breaking change)
• Breaking change (fix or feature with breaking changes)
• Documentation update

Breaking Changes

Test Plan

• Unit tests added/updated
• Integration tests added/updated
• Passed make installcheck
• Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

• Followed contribution guide
• Added/updated documentation
• Reviewed code for security implications
• Requested review from cloudberry committers

Additional Context

CI Skip Instructions

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[tuhaihe]

Hey @yjhjstz, good catch! BTW, can you add the commit body message? I know this is easy to understand, but for better context and to reach a broader audience, a commit body message is needed.

[tuhaihe]

LGTM. But the commit body message is needed.

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