fix pixi deprecation warnings

[plandes]

No description provided.

[Logiquo]

Could you explain what deprecation warnings you are fixing? e.g. what was the original warning message

[plandes]

Could you explain what deprecation warnings you are fixing? e.g. what was the original warning message

Sure, this fixes warning messages like this that show up in the log while running the CI tests:

running unit tests...
⚠️ Warning: Top-level 'channels' in [package.build] is deprecated. Please move to 'backend.channels'.
 WARN Encountered 1 warning while parsing the manifest:
  ⚠ The `project` field is deprecated. Use `workspace` instead.
    ╭─[/home/runner/work/PyHealth/PyHealth/pyproject.toml:82:1]
 81 │     # specifies properties for the whole workspace
 82 │ ╭─▶ [tool.pixi.project]
 83 │ │   channels = ["conda-forge"]
 84 │ │   platforms = ["osx-arm64", "linux-64", "linux-aarch64", "win-64"]
 85 │ ├─▶ preview = ["pixi-build"]
    · ╰──── replace this with 'workspace'
 86 │     
    ╰────

[Logiquo]

LGTM, thanks