fix: correct TEST_DEBUG retrieval in build.ps1

[lemeurherve]

This PR fixes a typo introduced in #396 preventing activating the debug of tests via (amongst others) an environment variable.

It also removes the duplicated declaration of the docker commands variables, lints the code by replacing double quotes around string without any variable by simple quotes so they're the same in all build.ps1 code, correctly reindents a small section of code and shortens an output log.

Extracted from:

• chore: use docker bake to generate docker compose file #415

Testing done

Local tests + CI

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