Conditionalize two new tests on the test-pypi feature (#18369)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
Both `build_backend::tool_uv_build_backend_without_build_backend` and
`build_backend::tool_uv_build_backend_wrong_build_backend` fail in
offline environments because they try to download a build backend from
PyPI.

This PR conditionalizes both tests on the existing `test-pypi` feature.

## Test Plan

<!-- How was it tested? -->
Applied as a downstream patch to Fedora’s `uv` package.

crates/uv/tests/it/build_backend.rs

@@ -1449,6 +1449,7 @@ fn build_with_all_metadata() -> Result<()> {
 /// Warn for cases where `tool.uv.build-backend` is used without the corresponding build backend
 /// entry.
 #[test]
+#[cfg(feature = "test-pypi")]
 fn tool_uv_build_backend_without_build_backend() -> Result<()> {
     let context = uv_test::test_context!("3.12");
 
@@ -1512,6 +1513,7 @@ fn tool_uv_build_backend_without_build_backend() -> Result<()> {
 /// Warn for cases where `tool.uv.build-backend` is used without the corresponding build backend
 /// entry.
 #[test]
+#[cfg(feature = "test-pypi")]
 fn tool_uv_build_backend_wrong_build_backend() -> Result<()> {
     let context = uv_test::test_context!("3.12");