mkarchroot: unshare mount namespace to avoid broken root chroot creation

While `extra-x86_64-build -c` is running `pacstrap` and has
`/var/lib/archbuild/extra-x86_64/root/run` mounted, another user logs
in, thus creating a new mountpoint `/run/user/$uid` that propagates into
`/var/lib/archbuild/extra-x86_64/root/run/user/$uid` leading to a broken
root chroot.

Successive `extra-x86_64-build -c` calls will result in cleanup errors:

```
$ extra-x86_64-build -c
==> Creating chroot for [extra] (x86_64)...
-> Deleting chroot copy 'root'...
rm: skipping '/var/lib/archbuild/extra-x86_64/root/run', since it's on a different device
rm: skipping '/var/lib/archbuild/extra-x86_64/root/run', since it's on a different device
==> ERROR: Working directory '/var/lib/archbuild/extra-x86_64/root' already exists
==> ERROR: Aborting...
```

Reported by and patch adjusted from archlinuxcn.

Fixes FS#64698

0 files changed, 0 insertions, 0 deletions