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

1 files changed, 1 insertions, 1 deletions

diff --git a/mkarchroot.in b/mkarchroot.in
index 3e08af2..8fb52ac 100644
--- a/mkarchroot.in
+++ b/mkarchroot.in
@@ -81,7 +81,7 @@ for file in "${files[@]}"; do
 	cp "$file" "$working_dir$file"
 done
 
-pacstrap -${umode}Mcd ${pac_conf:+-C "$pac_conf"} "$working_dir" \
+unshare --mount pacstrap -${umode}Mcd ${pac_conf:+-C "$pac_conf"} "$working_dir" \
 	"${cache_dirs[@]/#/--cachedir=}" "$@" || die 'Failed to install all packages'
 
 printf '%s.UTF-8 UTF-8\n' en_US de_DE > "$working_dir/etc/locale.gen"