Fix a bug where no warning is output when parsing of workspace settings fails.

[Di-Is]

Summary

See #4013

uv pip ... command loads workspace settings from pyproject.toml and uv.toml.

Although a warning is implemented to output a warning when parsing fails, it is not actually output.

https://github.com/astral-sh/uv/blob/main/crates/uv-workspace/src/workspace.rs#L38-L61

The reason is that the flag to display warnings is enabled after loading the workspace settings.

This PR turns on the warning output flag before loading the workspace.

Test Plan

pyproject.toml for test

[project]
name = "sample"
version = "0.0.0"
dependencies = ["ruff"]

[tool.uv.pip]
# originally string type.
index-url = 1

command output (before modification)

uv pip compile pyproject.toml 

Resolved 1 package in 383ms
# This file was autogenerated by uv via the following command:
#    uv pip compile pyproject.toml
ruff==0.4.7
    # via sample (pyproject.toml)

command output (after modification)

uv pip compile pyproject.toml

warning: Failed to parse `pyproject.toml`: TOML parse error at line 7, column 13
  |
7 | index-url = true
  |             ^^^^
invalid type: boolean `true`, expected a string

Resolved 1 package in 107ms
# This file was autogenerated by uv via the following command:
#    uv pip compile pyproject.toml
ruff==0.4.7
    # via sample (pyproject.toml)

[konstin]

Thanks

[konstin]

CC @charliermarsh for the changes in cli-warnings handlings.

[charliermarsh]

We should eventually change this to buffer and then print.