make docbuilder work with old index as fallback (& add admin acct to trustees)

Hi, I'm experimenting with a private Hackage server at the moment, and I'm struggling to get doc building working.

I tried running `hackage-build` locally as `rundocs.sh` does, but I see this output:

```
There are 1 packages with a total of 1 package versions
So far we have built or attempted to built 0 packages; only 1 left!
Considering the most recent version only:
0 built succesfully
0 failed to build
1 not yet built
Considering all versions:
0 all versions built successfully
0 attempted to build all versions, but some failed
0 not all versions built yet, but those that did were ok
0 not all versions built yet, and some failures
1 no versions built yet
0 package(s) to build
```

I dug a little bit deeper, and it appears `hackage-build` is grabbing package indexes from `~/.cabal/config/packages` here:

https://github.com/haskell/hackage-server/blob/af9096e00bf251c6919e568b0d12441e6715cb65/exes/BuildClient.hs#L160

So, this explains why I got `0 package(s) to build` - the package uploaded to my private hackage is called `kab`, which does not exist on `hackage.haskell.org`.

Any advice on how to proceed here is appreciated!