[3.0] fetcher: don't download network metadata if the user doesn't want to

The option is defined as "Allow metadata network access". If it's false (default but query during the first run) that means the user doesn't want to check metadata from the network. So we should not start a network job when there's a possibility to do it.

This is a regression from ea88b8d6. Before that we only used FETCHER_SCOPE_LOCAL if the option wasn't set.

(cherry picked from commit 1a85d679)

Draft until !6170 (merged) merged in master.

The 3.0 code is more tricky so we just keep checking at the last minute we may want to access the network.