installation/guides/chroot: fix ROOTFS method permissions

[swomf]

The tar command used in the "ROOTFS method" heading loses important permissions data.

For example, unix_chkpwd (a program that uses the setUID bit to allow some lockscreen programs to talk to PAM) may have missing setUID permissions, leading to user lockout.

Using only tar xvf

# ls -l /mnt/usr/bin/unix_chkpwd
-rwxr-xr-x 1 root root 38880 Dec 26  2023 /mnt/usr/bin/unix_chkpwd

Using Gentoo-style tar extraction

# ls -l /mnt/usr/bin/unix_chkpwd
-rwsr-xr-x 1 root root 38880 Dec 26  2023 /mnt/usr/bin/unix_chkpwd

This commit updates the documentation's tar command to match how Gentoo unpacks their tarballs.

[swomf]

This is essentially a one-line change. It's odd that the build failed earlier but it should be fixed now.

[classabbyamp]

p, --preserve-permissions, --same-permissions Set permissions of extracted files to those recorded in the archive (default for superuser).

p shouldn't be necessary, as the command should be run as root (indicated by the # prompt)

as for --xattrs-include, I don't think there should be anything in the rootfs that has xattrs to preserve

numeric owner also shouldn't be necessary because even if the non-root ownership of a file isn't preserved, all files from packages with non-root ownership are given that ownership by a script that runs during the package's configuration, which is run during the installation process with xbps-reconfigure -fa