This reverts merge request !1027

This makes the output to be more compact while not loosing readability
--format=flycheck // --format=json could also be used but they are
not very human readable

Not really sure about the order in which this is but it feels right
because credo is more about checking if a ~working code could need
some improvements.<M-1>

[CI] Set MIX_ENV to test in `variables` section instead of specifying it before the command. Fix `mix compile` compiling in the dev env.