Always add bin path to $PATH on macos builders

[alopezz]

What does this PR do?

Moves the place where ${prefix}/bin is added to $PATH from the builder setup script to the script that drives the build.

Proof that the change works on cache hits: https://github.com/DataDog/integrations-core/actions/runs/12932489195/job/36068842241?pr=19466#step:7:19.

Motivation

The current implementation makes the build fail on cache hits, as the action of setting the PATH is not part of the cache.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged