sc-tracing: swap atty with is-terminal (#2718)

`atty` is unmaintaned. See the advisory
https://github.com/advisories/GHSA-g98v-hv3f-hcfr

I picked is-terminal because rustix is already in-tree, so doesn't
increase the dependency footprint, and I am not sure if we can rely on
`IsTerminal` from the std because I am not sure what our MSRV.