scaleway: switched credentials reading order

[Mia-Cross]

This PR switches the reading order of the two credentials sources to be consistent with the other Scaleway DevTools (see here).

From now on, the profile will be loaded first if it exists, and then the environment variables will be read, overriding the values in the profile.

This change is already documented in #15604

[k8s-ci-robot]

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[hakman]

/ok-to-test

[k8s-ci-robot]

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