[tlbuild] tl release candidate rebuild

[Karl Berry]

    'making ConTeXt MkIV cache with luatools --generate, be patient...'

1) context mkiv requires that cache.
2) context always looks for it in the user-specific TEXMFVAR.
3) if you want to change it, you can change TEXMFVAR in install-tl.
4) your msg makes me realize that we should mention this more prominently.