Log netrc parsing error (astral-sh#8364)

## Summary

Resolves astral-sh#7685

## Test Plan

```console
$ echo "this is an invalid netrc" > .netrc
$ NETRC=.netrc cargo run -- pip install anyio --index-url https://pypi-proxy.fly.dev/basic-auth/simple --strict -v
DEBUG uv 0.4.24 (f4d5fba 2024-10-19)
DEBUG Searching for default Python interpreter in system path or `py` launcher
DEBUG Found `cpython-3.11.2-windows-x86_64-none` at `D:\Projects\Rust\uv\.venv\Scripts\python.exe` (virtual environment)
DEBUG Using Python 3.11.2 environment at .venv
DEBUG Acquired lock for `.venv`
DEBUG At least one requirement is not satisfied: anyio
DEBUG Using request timeout of 30s
DEBUG Solving with installed Python version: 3.11.2
DEBUG Solving with target Python version: >=3.11.2
DEBUG Adding direct dependency: anyio*
DEBUG No cache entry for: https://pypi-proxy.fly.dev/basic-auth/simple/anyio/
WARN Error reading netrc file: parsing error: bad toplevel token 'this' (line 1) in the file '.netrc'
DEBUG Searching for a compatible version of anyio (*)
DEBUG No compatible version found for: anyio
  × No solution found when resolving dependencies:
  ╰─▶ Because anyio was not found in the package registry and you require anyio, we can conclude that your
      requirements are unsatisfiable.

      hint: An index URL (https://pypi-proxy.fly.dev/basic-auth/simple) could not be queried due to a lack of valid
      authentication credentials (401 Unauthorized).
DEBUG Released lock at `D:\Projects\Rust\uv\.venv\.lock`
error: process didn't exit successfully: `target\debug\uv.exe pip install anyio --index-url https://pypi-proxy.fly.dev/basic-auth/simple --strict -v` (exit code: 1)

```

crates/uv-auth/src/middleware.rs

@@ -1,4 +1,4 @@
-use std::sync::Arc;
+use std::sync::{Arc, LazyLock};
 
 use http::{Extensions, StatusCode};
 use url::Url;
@@ -12,14 +12,48 @@ use anyhow::{anyhow, format_err};
 use netrc::Netrc;
 use reqwest::{Request, Response};
 use reqwest_middleware::{Error, Middleware, Next};
-use tracing::{debug, trace};
+use tracing::{debug, trace, warn};
+
+/// Strategy for loading netrc files.
+enum NetrcMode {
+    Automatic(LazyLock<Option<Netrc>>),
+    Enabled(Netrc),
+    Disabled,
+}
+
+impl Default for NetrcMode {
+    fn default() -> Self {
+        NetrcMode::Automatic(LazyLock::new(|| match Netrc::new() {
+            Ok(netrc) => Some(netrc),
+            Err(netrc::Error::Io(err)) if err.kind() == std::io::ErrorKind::NotFound => {
+                debug!("No netrc file found");
+                None
+            }
+            Err(err) => {
+                warn!("Error reading netrc file: {err}");
+                None
+            }
+        }))
+    }
+}
+
+impl NetrcMode {
+    /// Get the parsed netrc file if enabled.
+    fn get(&self) -> Option<&Netrc> {
+        match self {
+            NetrcMode::Automatic(lock) => lock.as_ref(),
+            NetrcMode::Enabled(netrc) => Some(netrc),
+            NetrcMode::Disabled => None,
+        }
+    }
+}
 
 /// A middleware that adds basic authentication to requests.
 ///
 /// Uses a cache to propagate credentials from previously seen requests and
 /// fetches credentials from a netrc file and the keyring.
 pub struct AuthMiddleware {
-    netrc: Option<Netrc>,
+    netrc: NetrcMode,
     keyring: Option<KeyringProvider>,
     cache: Option<CredentialsCache>,
     /// We know that the endpoint needs authentication, so we don't try to send an unauthenticated
@@ -30,19 +64,23 @@ pub struct AuthMiddleware {
 impl AuthMiddleware {
     pub fn new() -> Self {
         Self {
-            netrc: Netrc::new().ok(),
+            netrc: NetrcMode::default(),
             keyring: None,
             cache: None,
             only_authenticated: false,
         }
     }
 
-    /// Configure the [`Netrc`] credential file to use.
+    /// Configure the [`netrc::Netrc`] credential file to use.
     ///
     /// `None` disables authentication via netrc.
     #[must_use]
-    pub fn with_netrc(mut self, netrc: Option<Netrc>) -> Self {
-        self.netrc = netrc;
+    pub fn with_netrc(mut self, netrc: Option<netrc::Netrc>) -> Self {
+        self.netrc = if let Some(netrc) = netrc {
+            NetrcMode::Enabled(netrc)
+        } else {
+            NetrcMode::Disabled
+        };
         self
     }
 
@@ -361,7 +399,7 @@ impl AuthMiddleware {
         }
 
         // Netrc support based on: <https://github.com/gribouille/netrc>.
-        let credentials = if let Some(credentials) = self.netrc.as_ref().and_then(|netrc| {
+        let credentials = if let Some(credentials) = self.netrc.get().and_then(|netrc| {
             debug!("Checking netrc for credentials for {url}");
             Credentials::from_netrc(
                 netrc,