env: add a flag to output as environment variable bindings

[rjbou]

fix #5791

The name is still to be determined (it's a sed-able name ftm)

[kit-ty-kate]

I'm ok with the idea but i'm really not sure about the name of the new option.

--bindings(s) doesn't really describe anything to me personally. I haven't thought about it much but so far i prefer the original suggestion: --bare.

One other issue is that the expected usage is to add to the $GITHUB_ENV file in Github Actions. However the syntax for it is slightly more complex than we can first assume. In the rare case where a newline character is part of the value of an environment variable, the "binding" syntax is not correct anymore.
I think we should check that no newline character exists before printing and fail otherwise.

[rjbou]

From dev meeting: Consensus over the name --raw. We won't handle newline in this PR, but prefer output a constructed json for CI handling.

[rjbou]

Update with --raw

[rjbou]

From (last week) dev meeting: we are going to a full json output for CI, containing the information of the full computed path, and also what to append and/or prepend. Like that CI scripts can use that information to change their environment variables as needed.
For ex:

{ 
  "var" : "PATH"
  "full": "foo:bar:/usr/bin:/etc:baz";
  "prepend": ["foo"; "bar"];
  "append": ["baz"]
}