Backport v2023 <- v2025pre: Disable stdbuf

The stdbuf build process involves building a shared library externally and embedding it in the binary, which makes it difficult to extract debug info from binaries and then still strip them. This is pretty hard to fix while keeping coreutils cargo-installable, so just disable stdbuf instead.

infrastructure/apertis-issues#431 (closed)

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com