By unsetting CXXFLAGS in order to build Lagom code generators without
instrumentation, we were also removing -stdlib=libc++, so Clang ended
up using the runner image's libstdc++ 9 headers. These are too old, and
don't contain <coroutine>, leading to a build failure.
By unsetting
CXXFLAGSin order to build Lagom code generators without instrumentation, we were also removing-stdlib=libc++, so Clang ended up using the runner image's libstdc++ 9 headers. These are too old, and don't contain<coroutine>, leading to a build failure.Fixes #24578