Clear discovered interpreters when creating virtual environment (#15522)

## Summary

Closes https://github.com/astral-sh/uv/issues/15518.

crates/uv-python/src/interpreter.rs

@@ -106,6 +106,7 @@ impl Interpreter {
             sys_prefix: virtualenv.root,
             target: None,
             prefix: None,
+            site_packages: vec![],
             ..self
         }
     }

crates/uv/tests/it/run.rs

@@ -5958,3 +5958,84 @@ fn run_python_preference_no_project() {
     ----- stderr -----
     ");
 }
+
+/// Regression test for: <https://github.com/astral-sh/uv/issues/15518>
+#[test]
+fn isolate_child_environment() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! { r#"
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        requires-python = ">=3.12"
+        dependencies = [
+          "iniconfig"
+        ]
+
+        [tool.uv.workspace]
+        members = ["child"]
+        "#
+    })?;
+
+    context
+        .temp_dir
+        .child("child")
+        .child("pyproject.toml")
+        .write_str(indoc! { r#"
+        [project]
+        name = "child"
+        version = "1.0.0"
+        requires-python = ">=3.12"
+        dependencies = []
+        "#
+        })?;
+
+    // Sync the parent package.
+    uv_snapshot!(context.filters(), context.sync(), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    ");
+
+    // Ensure that the isolated environment can't access `iniconfig` (from the parent package).
+    uv_snapshot!(context.filters(), context.run().arg("--package").arg("child").arg("--isolated").arg("python").arg("-c").arg("import iniconfig"), @r#"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Audited in [TIME]
+    Traceback (most recent call last):
+      File "<string>", line 1, in <module>
+    ModuleNotFoundError: No module named 'iniconfig'
+    "#);
+
+    // Ensure that the isolated environment can't access `iniconfig` (from the parent package).
+    uv_snapshot!(context.filters(), context.run().arg("--package").arg("child").arg("--isolated").arg("--with").arg("typing-extensions").arg("python").arg("-c").arg("import iniconfig"), @r#"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Audited in [TIME]
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + typing-extensions==4.10.0
+    Traceback (most recent call last):
+      File "<string>", line 1, in <module>
+    ModuleNotFoundError: No module named 'iniconfig'
+    "#);
+
+    Ok(())
+}