Re-instate use of pyenv

[jamesreprise]

What is the goal of this PR?

We've re-instated the use of pyenv in certain Factory job definitions. Removing it in favour of using the system python3 had far-reaching ramifications that proved too costly to immediately address.

What are the changes implemented in this PR?

We've re-instated our use of pyenv in deploy-github, deploy-apt-snapshot and deploy-apt-release.

[typedb-bot]

PR Review Checklist

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Code

• [x] Packages, classes, and methods have a single domain of responsibility.
• [x] Packages, classes, and methods are grouped into cohesive and consistent domain model.
• [x] The code is canonical and the minimum required to achieve the goal.
• [x] Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
• [x] Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
• [x] The code is algorithmically efficient and scalable for the whole application.

Architecture

• [x] Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
• [x] Any required build and release automations are updated and/or implemented.
• [x] Any new components follows a consistent style with respect to the pre-existing codebase.
• [x] The architecture intuitively reflects the application domain, and is easy to understand.
• [x] The architecture has a well-defined hierarchy of encapsulated components.
• [x] The architecture is extensible and scalable.