unit-test: add workaround for clang-20 errors

We are seeing pervasive issues with clang-20 with boost async_wait
calls mixed with std::bind_front:

```
/usr/local/include/sdbusplus/asio/connection.hpp:411:15: error: no matching member function for call to 'async_wait'
  411 |         timer.async_wait(std::bind_front(&connection::on_timer_event, this));
```

This issue is noted upstream[1] and is currently unsolved, but we
discovered we can solve this by passing `-fno-builtin-std-forward_like`
as a clang compiler option.  Add this to the unit-test scripts when
the `CXX` is clang.

[1]: https://github.com/llvm/llvm-project/issues/101614

Tested: Ran locally against dbus-sensors and observed the compile
issues are resolved.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I3e5a68320df10fd835613da452c0fcefa95e6a22
diff --git a/scripts/unit-test.py b/scripts/unit-test.py
index 8f28ee0..1b3d1f5 100755
--- a/scripts/unit-test.py
+++ b/scripts/unit-test.py
@@ -1062,7 +1062,10 @@
         # Run clang-tidy only if the project has a configuration
         if os.path.isfile(".clang-tidy"):
             os.environ["CC"] = "clang"
-            os.environ["CXX"] = "clang++"
+            # Clang-20 currently has some issue with libstdcpp's
+            # std::forward_like which results in a bunch of compile errors.
+            # Adding -fno-builtin-std-forward_like causes them to go away.
+            os.environ["CXX"] = "clang++ -fno-builtin-std-forward_like"
             with TemporaryDirectory(prefix="build", dir=".") as build_dir:
                 check_call_cmd("meson", "setup", build_dir)
                 if not os.path.isfile(".openbmc-no-clang"):