After descriptors overhaul, the gitian VM tries to access network - need workaround

[laanwj]

It should be possible to build in a gitian VM without access the internet. At the least there needs to be a documented workaround for this (maybe this exists but we just didn't find it).

@sipa tried this yesterday and didn't succeed in building, even after manually triggering download and copying the cache, ie

cd bitcoin/depends
make download
cp sources/* $GITIAN_PATH/cache/common

... but the builder still tried to download e.g. the .jar files..

[theuni]

ACK. This is do-able right now, but it's a clunky manual process. I'll add a straightforward method and documentation for it.