do not require if config.py if using local metadata

do not require if config.py if using local metadata

Since you can have a .fdroid.* metadata file included directly in an
app's git repo, it seems annoying to force developers to also include
a blank config.py in order to run `fdroid build` from their git repo.

The next step after this is some kind of global config file for
setting paths and things that need to be customized that would apply
to all git repos, for example, to enable using a buildserver.

This also required moving all of the env setting for FDroidPopen into
a single place so that it is properly setup when using the local
metadata.