lttng-ust has a mandatory dependency on liburcu, which requires
architecture-specific code that isn't available for all platforms. To
enable building libcamera for those platforms, disable tracing support
if lttng-ust is unavailable. This fixes building libcamera for, for
example, loongarch64-linux.
The CI check complains about a new attribute introduced that's not using
the by-name pattern, even though it's using the same .nix file.
It seems using callPackage ../by-name/… from toplevel for the
alternative toplevel attribute seems to be others do this, so using the
same pattern here.
