Store build logs in /nix/var/log/nix/drvs/<XX>

...where <XX> is the first two characters of the derivation.
Otherwise /nix/var/log/nix/drvs may become so large that we run into
all sorts of weird filesystem limits/inefficiences.  For instance,
ext3/ext4 filesystems will barf with "ext4_dx_add_entry:1551:
Directory index full!" once you hit a few million files.
corepkgs/fetchurl: Enable making the downloaded file executable
build-remote.pl: Don't keep build logs on the build slave
Fix the VM tests
Correctly handle missing logs
Only warn about SQLite being busy once

No need to get annoying.