Fix a bug where no warning is output when parsing of workspace settings fails. (#4014)

## 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

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

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

command output (before modification)
```bash
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)

```bash
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)
```

crates/uv-warnings/src/lib.rs

@@ -17,6 +17,11 @@ pub fn enable() {
     ENABLED.store(true, std::sync::atomic::Ordering::SeqCst);
 }
 
+/// Disable user-facing warnings.
+pub fn disable() {
+    ENABLED.store(false, std::sync::atomic::Ordering::SeqCst);
+}
+
 /// Warn a user, if warnings are enabled.
 #[macro_export]
 macro_rules! warn_user {