nfsconvert.sh: Stop using the immutable bit
The immutable bit is a bad idea; it's fairly obscure - many Linux
administrators won't know about it.
Further, it actively breaks ostree, which maintains multiple root
filesystems: https://discussion.fedoraproject.org/t/boot-partition-of-silverblue-is-without-space/771/12
Instead let's just `chmod -w` - however of course this doesn't
actually have an effect for `root` because of `CAP_DAC_OVERRIDE`
but we're doing what we can.