makepkg.conf: enable rust debuginfo
This enables DEBUG_RUSTFLAGS with the appropriate debuginfo settings.
The empty RUSTFLAGS variable is required workaround to avoid double compilation during package function call. The issue is that the behavior of the current implementation of buildenv_debugflags is not idempotent, so consecutive calls will append the same flags again leading to cargo consider the build inputs to have changed.
Signed-off-by: Levente Polyak anthraxx@archlinux.org