Allow system Python discovery with --target and --prefix (#9371)

## Summary

If we're installing with `--target` or `--prefix`, then it's not a
mutable operation, so we should be allowed to discover system Pythons. I
suspect this was hard to special-case in the past but is now trivial
after @zanieb's various refactors.

Closes #9356.

crates/uv/src/commands/pip/install.rs

@@ -138,17 +138,22 @@ pub(crate) async fn pip_install(
         )
         .collect();
 
+    // Determine whether we're modifying the discovered environment, or a separate target.
+    let mutable = !(target.is_some() || prefix.is_some());
+
     // Detect the current Python interpreter.
     let environment = PythonEnvironment::find(
         &python
             .as_deref()
             .map(PythonRequest::parse)
             .unwrap_or_default(),
-        EnvironmentPreference::from_system_flag(system, true),
+        EnvironmentPreference::from_system_flag(system, mutable),
         &cache,
     )?;
 
-    report_target_environment(&environment, &cache, printer)?;
+    if mutable {
+        report_target_environment(&environment, &cache, printer)?;
+    }
 
     // Apply any `--target` or `--prefix` directories.
     let environment = if let Some(target) = target {

crates/uv/src/commands/pip/sync.rs

@@ -122,17 +122,22 @@ pub(crate) async fn pip_sync(
         }
     }
 
+    // Determine whether we're modifying the discovered environment, or a separate target.
+    let mutable = !(target.is_some() || prefix.is_some());
+
     // Detect the current Python interpreter.
     let environment = PythonEnvironment::find(
         &python
             .as_deref()
             .map(PythonRequest::parse)
             .unwrap_or_default(),
-        EnvironmentPreference::from_system_flag(system, true),
+        EnvironmentPreference::from_system_flag(system, mutable),
         &cache,
     )?;
 
-    report_target_environment(&environment, &cache, printer)?;
+    if mutable {
+        report_target_environment(&environment, &cache, printer)?;
+    }
 
     // Apply any `--target` or `--prefix` directories.
     let environment = if let Some(target) = target {

crates/uv/tests/it/common/mod.rs

@@ -315,7 +315,7 @@ impl TestContext {
         // Exclude `link-mode` on Windows since we set it in the remote test suite
         if cfg!(windows) {
             filters.push(("--link-mode <LINK_MODE> ".to_string(), String::new()));
-            filters.push(((r#"link-mode = "copy"\n"#).to_string(), String::new()));
+            filters.push((r#"link-mode = "copy"\n"#.to_string(), String::new()));
         }
 
         filters.extend(

crates/uv/tests/it/pip_sync.rs

@@ -5361,6 +5361,36 @@ fn target_no_build_isolation() -> Result<()> {
     Ok(())
 }
 
+/// Sync to a `--target` directory without a virtual environment.
+#[test]
+fn target_system() -> Result<()> {
+    let context = TestContext::new_with_versions(&["3.12"]);
+
+    // Install `iniconfig` to the target directory.
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str("iniconfig==2.0.0")?;
+
+    uv_snapshot!(context.filters(), context.pip_sync()
+        .arg("requirements.in")
+        .arg("--target")
+        .arg("target"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    "###);
+
+    // Ensure that the package is present in the target directory.
+    assert!(context.temp_dir.child("target").child("iniconfig").is_dir());
+
+    Ok(())
+}
+
 /// Sync to a `--prefix` directory.
 #[test]
 fn prefix() -> Result<()> {