Modify alembic function for virtual environment support

[jwlademann]

Source venv if env var is defined to allow support for newer images (e.g. python 3.13)
Something of a duplicate of #182 since Sarah is off sick currently and we're hitting this issue with our new APIs.

• What kind of change does this PR introduce (Bug fix, feature, docs update, ...)?
  Source venv if env var is defined to allow support for newer images (e.g. python 3.13)

• What is the current behavior?
  Alembic alias does not work with newer base images, e.g. python 3.13

• What is the new behavior (if this is a feature change)?
  Fixes existing feature

• Does this PR introduce a breaking change? If so, what actions will users need to take in order to regain compatibility?
  No

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