Fix tag variable in Publish Image step

[karthick-rn]

Description

Fix tag variable initialisation in Publish Image step to properly extract and use tag name from git reference.

Addresses: https://github.com/microsoft/planetary-computer-apis/actions/runs/19644058576/job/56348155639#annotation:9:17

Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Checklist:

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• I have performed a self-review
• Changelog has been updated
• Documentation has been updated
• Unit tests pass locally (./scripts/test)
• Code is linted and styled (./scripts/format)