fix(back): correct AWS env var name

[BertrandRenault]

close #137

[timothee-LJN]

to keep consistency and follow the conventional commit pattern you could rename the PR
chore: Correct AWS env var names

[cpvannier]

Pull request name should be updated to follow the Conventional Commits guidelines.

Also, something seems wrong with the name of issue #137 reporting the error and the steps to reproduce:
This looks like a problem caused by #93 not updating the serve.py file, causing a conflict between the App args and the Settings args. But #93 is not included in v0.5.0b3, so this error does not appear when running pip install pixano==0.5.0b3, only when installing Pixano from the develop branch (locally or from GitHub).

[BertrandRenault]

You right, it was not from pixano0.5.0b3 but local develop version. Issue has been edited accordingly