update_chroot: Fix /etc/portage/patches symlink

[chewi]

This was accidentally broken by #3795. I took that change from a branch where the COREOS_OVERLAY variable was no longer used at all.

How to use

In the SDK:

$ ls -l /etc/portage/patches 
lrwxrwxrwx 1 root root 20 Apr 15 11:07 /etc/portage/patches -> /coreos/user-patches

$ ./update_chroot --setuponly

$ ls -l /etc/portage/patches 
lrwxrwxrwx 1 root root 67 Apr 15 11:13 /etc/portage/patches -> /mnt/host/source/src/third_party/coreos-overlay/coreos/user-patches

Testing done

I've done the above.

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update) -- N/A
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.