Update sources after sources.list update

I saw the following error:

<SNIP>
Set sources list to:
+ cat /tmp/tmpp6f7tW/etc/apt/sources.list
deb http://deb.debian.org/debian buster main
<SNIP>
+ chroot /tmp/tmpp6f7tW apt-get install -y gnupg
Reading package lists... Done
Building dependency tree... Done
Package gnupg is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gnupg' has no installation candidate

Could be because I went and added my local apt-cache proxy to 'mirror=' in conf/dists/buster?