[tlbuild] tl release candidate rebuild

[Karl Berry]

    But is it sensible to try generating this cache at install-tl time? 

When else?  As I understand it, ctx is not prepared to generate it on
the fly, but maybe I am wrong.

    a cache in root's home will probably not help

Certainly, but we also don't recommend installing TL as root.
Not up to us to second-guess the installer.
(This is a side benefit I didn't think about when I added setting
TEXMFVAR as an option in install-tl.)