Chroot builds: Provide world-readable /nix/store

This was causing NixOS VM tests to fail mysteriously since
5ce50cd99e. Nscd could (sometimes) no
longer read /etc/hosts:

open("/etc/hosts", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)

Probably there was some wacky interaction between the guest kernel and
the 9pfs implementation in QEMU.

src/libstore/build.cc

@@ -1895,7 +1895,7 @@ void DerivationGoal::startBuilder()
            build user. */
         Path chrootStoreDir = chrootRootDir + settings.nixStore;
         createDirs(chrootStoreDir);
-        chmod_(chrootStoreDir, 0730);
+        chmod_(chrootStoreDir, 0735);
 
         if (chown(chrootStoreDir.c_str(), 0, buildUser.getGID()) == -1)
             throw SysError(format("cannot change ownership of ‘%1%’") % chrootStoreDir);