cabal new-build --package-db=foo doesn't do anything

[phadej]

if foo doesn't exist: builds
if foo exists: doesn't consider it.

I think that for 3.0 we should either remove package database flags from new-build commands; or teach these commands to use other package databases. I.e. to consider other, not only global database. I actually have a use case for this. ("obviously", if I use some weird --package-db, I'd use my own --store-dir etc. so this is intended for people who know what they are doing).