master: Don't depend unconditionally on xattr in darwin_install

As far as I know, the 8.10 series runs xattr starting with 8.10.5 as part of the build, as well as the GHC 9.2 series and the master branch. However there has been a 8.10.x-only change to make it possible to override the xattr used via the XATTR environment variable in ec451cac. It would be nice to have this in master as well and backported to 9.2.1 (I don't know the process for that, so I'll just open a MR for master for now).

The 8.10 change applies on master as well, so I've just cherry-picked it.