Use chroots for all derivations

If ‘build-use-chroot’ is set to ‘true’, fixed-output derivations are
now also chrooted. However, unlike normal derivations, they don't get
a private network namespace, so they can still access the
network. Also, the use of the ‘__noChroot’ derivation attribute is
no longer allowed.

Setting ‘build-use-chroot’ to ‘relaxed’ gives the old behaviour.

src/libstore/globals.hh

@@ -145,9 +145,6 @@ struct Settings {
     /* The Unix group that contains the build users. */
     string buildUsersGroup;
 
-    /* Whether to build in chroot. */
-    bool useChroot;
-
     /* Set of ssh connection strings for the ssh substituter */
     Strings sshSubstituterHosts;