Split package build and deployment into separate tasks in the pipeline

[kieran-ryan]

The project is currently packaged and published in each of the deployment jobs: to PyPI and TestPyPI. This slows down the pipeline as the package build process runs twice. As the project is only built on release - when the package would be deployed - build failures are not detected in the pipeline until a release is created.

To resolve these issues, the build process can be extracted from the deployment tasks into its own individual task, of which other tasks can use its distribution.