On macOS systems with Homebrew installed, the latter will install its
own copy of `xattr` in `/opt/homebrew/bin/xattr` which will often
take precedence over the system `xattr` at `/usr/bin/xattr`, and does
not support the `-r` flag to act recursively over a directory.
This commit changes the invocation of `xattr` to use the absolute path
to the system version of `xattr` at `/usr/bin/xattr`.