Apply system Python filtering to executable name requests (#4309)

Executable name requests were being treated as explicit requests to
install into system environments, but I don't think it should be as it's
implicit what environment you'll end up in. Following #4308, we allow
multiple executables to be found so we can filter here.

Concretely, this means `--system` is required to install into a system
environment discovered with e.g. `--python=python`. The flag is still
not required for cases where we're not mutating environment.

crates/uv-python/src/discovery.rs

@@ -717,11 +717,15 @@ pub fn find_python_installations<'a>(
             if preference.allows(PythonSource::SearchPath) {
                 debug!("Checking for Python interpreter at {request}");
                 Box::new(
-                    python_interpreters_with_executable_name(name, cache).map(|result| {
-                        result
-                            .map(PythonInstallation::from_tuple)
-                            .map(FindPythonResult::Ok)
-                    }),
+                    python_interpreters_with_executable_name(name, cache)
+                        .filter(move |result| {
+                            result_satisfies_environment_preference(result, environments)
+                        })
+                        .map(|result| {
+                            result
+                                .map(PythonInstallation::from_tuple)
+                                .map(FindPythonResult::Ok)
+                        }),
                 )
             } else {
                 Box::new(std::iter::once(Err(Error::SourceNotAllowed(

crates/uv-python/src/lib.rs

@@ -1609,6 +1609,48 @@ mod tests {
             "We should find the `bar` executable"
         );
 
+        // With [`EnvironmentPreference::OnlyVirtual`], we should not allow the interpreter
+        let result = context.run(|| {
+            find_python_installation(
+                &PythonRequest::parse("bar"),
+                EnvironmentPreference::ExplicitSystem,
+                PythonPreference::OnlySystem,
+                &context.cache,
+            )
+        })?;
+        assert!(
+            matches!(result, Err(PythonNotFound { .. })),
+            "We should not allow a system interpreter; got {result:?}"
+        );
+
+        // Unless it's a virtual environment interpreter
+        let mut context = TestContext::new()?;
+        let python = context.tempdir.child("foo").join("bar");
+        TestContext::create_mock_interpreter(
+            &python,
+            &PythonVersion::from_str("3.10.0").unwrap(),
+            ImplementationName::default(),
+            false, // Not a system interpreter
+        )?;
+        context.add_to_search_path(context.tempdir.child("foo").to_path_buf());
+
+        let python = context
+            .run(|| {
+                find_python_installation(
+                    &PythonRequest::parse("bar"),
+                    EnvironmentPreference::ExplicitSystem,
+                    PythonPreference::OnlySystem,
+                    &context.cache,
+                )
+            })
+            .unwrap()
+            .unwrap();
+        assert_eq!(
+            python.interpreter().python_full_version().to_string(),
+            "3.10.0",
+            "We should find the `bar` executable"
+        );
+
         Ok(())
     }