Deprecate matrix builds in favor of array arguments

[idelvall]

It would be useful to have a way to pass an array as an argument, without requiring the developer to deal with its serialization/deserialization.

In particular, I would like that repeated args are available as a bash array:

foo:
  DO +BAR \
    --color=red
    --color=blue
    --color=green
  DO +BAR2 \
    --color=red
    --color=blue
    --color=green

BAR:
  COMMAND
  ARG color
  RUN \
    for c in "${color[@]}"; \
      do echo $c; \
    done

BAR2:
  COMMAND
  ARG color
  FOR c IN $color
    RUN echo $c
  END

Right now, arg repetition makes the target being called multiple times in parallel (matrix builds), but I found this other behavior more intuitive.

Proposal

• Deprecate matrix builds in favor of array arguments.
• Feature flag this new behavior for a while, and whenever a matrix build is found (the feature flag is not present), warn the user about this behavior changing in the future.
• Express the matrix builds in terms of these arrays args (more readable IMO)

[vladaionescu]

Note that the proposed syntax is currently used to represent a matrix target.

This is likely a duplicate of https://github.com/earthly/earthly/issues/1890 (currently on the roadmap). Please reopen if not!

[idelvall]

Note that the proposed syntax is currently used to represent a matrix target.

I know, actually my proposal is to deprecate this behavior in favor of array arguments. Updating the description