skel.bashrc PS1 debian_chroot not dynamic in xterm title

https://git.launchpad.net/~iheino+ub/ubuntu/+source/bash/commit/?h=bugfix/1960366

Expected:

debian_chroot part of xterm title should update dynamically; with the following PS1 contents by default

$ set | fgrep PS1 | head -1
PS1='\[\e]0;${debian_chroot:+($debian_chroot)}\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '

Actual result without patch:

debian_chroot part of xterm title got evaluated only during bash startup, as can be seen in PS1 contents

$ set | fgrep PS1 | head -1
PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '