Fix PATH prepending when using Fish

[davidmartos96]

Support Linux on EnvMixin to handle fish_user_paths being prepended to the PATH, overriding any configuration in the VSCode environment via the context.environmentVariableCollection API.

Fixes #232495

(The root problem was originally fixed for macOS on #99878 but it can also trigger when using the fish shell in other platforms)

More context: #226589 (comment)

cc @meganrogge

[davidmartos96]

@Tyriar I applied the fix conditionally based on the shell being used.

While further testing it, I noticed that the envMixin should be applied even on non-login shells with fish, because the variable $fish_user_paths is always set, regardless if the shell is login or not.
Otherwise, in order for the fix to work, the user needs to explicitly use the following config. And that kind of extra configuration is not necessary for the PATH prefixing to work on both bash and zsh.

{
  "terminal.integrated.profiles.linux": {
    "fish login": {
      "path": "fish",
      "args": [
        "-l"
      ]
    }
  },
  "terminal.integrated.defaultProfile.linux": "fish login"
}

[Tyriar]

@meganrogge could you review this please? I've only done a very high level review not digging into the details.

[meganrogge]

Thanks for working on this. I created this issue - we'll want to link it to this PR. #232495

addressed