load: move env var profile detection to option

[milas]

Instead of reading COMPOSE_PROFILES environment variable directly during load(), rely exclusively on opts.Profiles.

A new loader option, WithDefaultProfiles, allows passing profile name(s) via varargs. If none are provided, it will fallback to using any profiles set via COMPOSE_PROFILES.

This improves purity of the loader and fixes issues with profiles being ignored for include (docker/compose#10906).

[milas]

Also, technically COMPOSE_PROFILES is not part of the spec (we don't specify HOW things like CLI should work).

At the same time, one thing we've learned from docker/cli is that it's really annoying if a bunch of conventions (DOCKER_CONTEXT) aren't supported by the "lower-level" client (moby/moby). So that's why I went with the WithDefaultProfiles loader option here - it keeps the logic in compose-go for the broader ecosystem, but it's also easy to avoid by using WithProfiles instead.

[milas]

@ndeloof This is slightly redundant now with your fix from #448 but I think it still makes sense to remove os.GetEnv logic from the loader itself, wdyt? Otherwise, I can close it

[ndeloof]

@milas loader definitively should not use os.GetEnv