In newer versions of Nix (at least on 2.4pre20201102_550e11f) the
`extra-` prefix for config options received a special meaning and the
option `extra-sandbox-paths` isn't recognized anymore. This commit fixes
it.
It doesn't cause a behavior change when using older versions of Nix but
does cause an extra newline to appear in the config, thus changing the
hash.
aeeee447bc